I've used watch dog for nearly 20 years and while Instapark does the job it has quirks. Given it half the price of the dog, its worth considering given it works well as a water sensor

Pros
good construction
very loud alarm, louder than the Dog. It pulls 167mAdc on alarm vs 17mAdc from the Dog
wired water sensor is easy to move and place
comes with 9v batteries
low battery beep as alert, none on the Dog

Cons
high standby current. It consumes 10.0 uAdc vs the 0.1 uAdc of the Dog. You can calculate how long it will last if you know your 9v mAh but overall this is higher operational cost as any battery change is about yearly. I use 9v NiMH and its economical to use a fresh batteries yearly and recharge the used one. Luckily this has a low bat chirp if it fails before a year. The Dog low power means a battery change is near expiry date on alkaline.
the sensor 'platform' is top heavy and can tip over if not placed right. You can stick it between some crevice to stay put or tape it in place or put more weight on the platform
The sensor requires more water below it to trip as it has a greater distance between the floor and the sensor, on the Dog, the sensors are far closer to floor. Some folks have put cotton or toilet paper between the sensor, which wicks water and makes it more sensitive, but I found on damp days that causes false triggering as the paper or cotton get moist.
Opening the case isn't described in the instructions. You have to press on two plastic latches to pull the rear case UP and away from the body
It comes with screws for the case, which is not needed since its a tight fit as is with the plastic latches but could be useful if the latches fail. On the Dog, the battery is easier replace as the cover slides off, and there is no latch to break as the unit ages, my Dogs are on duty for nearly 20 years
The plastic suction cup isn't a reliable mount, easier to just tie a cord to the cup and let this thing hang off something
there is no cord keeper, so if you don't need to remotely place the sensor the wires just hang around. On the Dog, the case is also the cord keeper

While I haven't had any more water leaks to put these sensors to a real-use test, a teaspoon of water on the floor quickly set off the alarm, which then stopped immediately upon picking it up. This convinced me enough to feel they can be trusted. I actually bought them as a backup to a Wasserstein wi-fi enabled sensor that also works but I question how reliable it will be given the difficulty getting it to work initially with the smartphone app. That device also has no standard alarm of sufficient volume to get my attention upstairs, if using these downstairs as I intend. However, it does notify me via my cellphone, so that's great where these sensors will not reach me if I'm out of the house. But, when home, they are great and should easily get my attention.

Heeding the suggestion of another reviewer, I improved sensitivity to a scant amount of water by simply folding a 1-inch square of a paper towel and tucking it into the bottom face of the sensor holder. As I wanted the sensor to stand flat upon the floor behind some cabinets where I couldn't easily see if it was properly positioned, I applied quite literally, a 4-cent fix: The sensors came with double-sided mounting foam to permit attaching the sensors or alarm box as desired. Cutting a piece of foam into 4 squares and affixing those squares at the four corners of the sensor holder's upper face, I then added a penny atop each square. This added enough weight to keep the sensor holder flat as I lowered it behind the cabinet. The wire was just long enough for the alarm box to simply lay discreetly at a back corner of the cabinet. At 3 for $20, these sensors are a "no-brainer" for anyone with water leak, flooding, overlflows, or other such issues.

I gave it a good overall score mostly based on the fact it works and you get three for the price, a good value. Noise level is decent, not as loud as my Nest fire/smoke alarm.

The battery case could have been made for easier access with a slip on/off cover like remote controls use. The case would be better with more battery room. The case would also benefit from a hang tab with a "snowman" hole allowing it to be hung by slipping over the head of a small nail or screw. The suction cup is pretty useless and the idea of gluing the back of the case to a wall with the adhesive pad is kinda dumb. You'd need to tear it off the wall to get to the hard to open cover.

1. Opening the case is easiest with two small screwdriver pushing back the tabs and a third to pry open.
2. Remove the one small screw holding the circuit board, connect the battery, slip the wires behind the divider, angle the battery in, nose first while gently pressing the rear end down. Yes, a very tight fit.
3. Place the circuit board over the two plastic pins and screw it down again (small Phillips).
4. Hook the top of the cover, press down the end with the two tabs (I colored then orange to more easily see).
5. I hot melt glued two steel nits to the sensor flange for better floor contact.
6. Placed sensor on paper towel "mat". Absorbs floor water like a sponge. Without the paper towel, the alarm activates when the floor water level exceed 1/16" high.
7. I used absorbent paper towel to make a wick to fill the sensor channel.
8. I spliced an 8 ft cord into mine to allow me to place the alarm case on ceiling ductwork to better amplify the already loud noisemaker. I used some 20 AWG wire, but you could use old telephone cable too. I soldered my spices and then used heat shrink tubing on the joints and another tube slipped over those.

When you receive the detectors (3 in a carton) and connect the provided 9V battery, it will operate as expected sounding a loud alarm when a leak is detected. I am happy with the purchase and would certainly recommend it. However that said, the device can be mechanically improved. Permit me to explain.

When you remove the detector from the first of three plastic bags, you need to open the back cover in order to insert the battery. Here is the first challenge since it is not clear how to open it (and the meager instructions do not address this). As you'll note from one of the photos, two "tabs" at one end of the unit when closed, fit into the slots. Thus to open the back, use a small flat-head screwdriver and push the tabs down so that they clear the slots. It's not easy the first time but thereafter it's not too bad and besides, you need to do this only when replacing the 9V battery (hopefully infrequently).

Once the back cover is opened, you'll immediately note the small plastic bag containing two small screws. So it seems the manufacturer may be suggesting to use the two screws if A) you wish to have an additional closure mechanism (can't imagine why), or B) if the tabs break off, using the screws in place of the tabs. I certainly did not use the screws and in fact, used a small file to try and reduce the size of the tabs so that the cover would open easier.

And by now you must have noticed the suction cup on the back cover used to attach the device to a surface. For me, the suction cup didn't work. Although the manufacturer provides a double-sided tape, this too is not a good solution since the tape will need to be ripped away whenever replacing the battery. A much better solution would be a piece of velcro on the back cover, or a "3M Company Re-Close Strip" (similar to the EZPASS attachment used on a car windshield), or gluing a small magnet to the back cover (thereby enabling attachment to a metal surface). With any of these options, the device can easily be removed and replaced without destroying the attachment itself.

So now that the back cover is open, the 9V battery can be connected, leading to another small challenge (as many have previously noted). The battery compartment is very tight making it difficult to place the connected battery into the compartment. However here's what worked for me. First connect the battery to the associated snaps. Second, ensure that the battery cable will be situated at the bottom of the compartment (not the top) when inserting the battery. Third, using your fingers reduce the width of the battery snaps as much as possible so that it's about the same width as the battery itself. Fourth, insert the battery into the compartment with the snaps end first and then press down on the other end to push it into the compartment. After you've done so once or twice, it becomes relatively easy. (Of course, had the manufacturer increased the case width by only an 1/8 inch, this would not be an issue.)

Having inserted the battery, the back cover needs to be closed, an easy operation, and the probe needs to be attached to the base (or platform), also an easy operation. At this point, I'd suggest you test the unit. To do so, simply moisten your index finger and for an instant slightly touch the probe. If should emits a loud shrill sound, indicating that the unit is functioning correctly. The device is now ready to be placed into operation, to the area where you wish to detect a water leak.

Now should you be interested and have the time, here are a couple of improvements I've made. They are not essential to the operation but I believe it will make the device more reliable and sensitive.

1st improvement: Because the probe and base assembly is so light weight, it may easily move around a bit and thus no longer lie flat on the surface, essential for optimum operation. To get around the light weight of the base base, you can glue on some material on either side of the base, such as screw nuts or as I used, dead AAA batteries (see photo). Thus by making the base heavier, once placed on the surface to be monitored, it will lie flat and not have the tendency to move around as easily.

2nd improvement: This improvement, actually suggested by other users (thank you) is in the sensitivity of the detector. The plastic of the base to which the probe is attached is about 1/32 inch in thickness. Therefore the detector will detect water accumulation only when it reaches about 1/32 inch. By placing a tissue that's been rolled and crunched up into the bottom center channel of the base (see photo), the first indication of water will be wicked up by the tissue and thus activate the audible alarm much sooner than if the water had to accumulate to 1/32 inch in height. However just be careful that the tissue lies totally flat to the base.

In summary, this is a very worthwhile and economical water leak detector. The mechanical issues described above should not detract you from purchasing the device. And once familiarized, these challenges become less of an issue the next time you deal with it. Finally it's important to note that the device relies heavily on the condition of the battery. It's prudent to check the battery condition on a regular basis, and possibly to replace it with a fresh one annually (or sooner if deemed necessary). Using masking tape or similar, I mark the date I last changed the battery on the case.

Good luck.... hope this helps.

Having recently received a 2nd order of units, this package cam along with a very reasonably sized and readable Instruction Manual (see photo). Evidently the manufacturer must be reading our comments...good! The most important part of this manual addresses the LOW BATTERY ALERT which reads as follows: "When the battery is low and needs to be replaced, you will hear an interval "beep" sound. Please replace battery with a new one." That said, I think it would still be wise to replace the battery annually.

In case of water seepage due to sump pump failure or water heater malfunction. Good product. Works as advertised. Nice and loud with a loud high pitched sound variating alarm that's much easier to hear. Our last one was not loud enough to even know it was going off. Great value. .