I want to extend the seasonal use of our swimming pool. This should be a good, cheap solution. I want it to work.

I cannot find the proper NEMA 6-15P 15 Amp 250 Volt external outlet on Amazon. And finding it in general is not straightforward. I'd prefer it as a GFCI outlet which I cannot find either. Because of the warning, I do not want to use a convertible extension cord which appears more readily available.
I like to have things on hand to save time and money with electricians, plumbers, etc.

I will continue to look at electrical and big box stores. Perhaps our electrician will be able to buy locally. If finding the electrical box is difficult, I now wonder how much the electrical installation will cost.
The box itself is labeled as a "Bathtub Heater".

It would have been so convenient if the seller would have included or referenced a couple of specific, compatible outlets. My project is temporarily flumoxed by a simple outlet.

I bought the plug adapter for being able to use this on my 100V outlet.

Then, I got one of those plastic sided ice bath cylinders and filled it with water.

I let this heater heat up the water and hopped in... after removing the heater, of course.

This is a great tool for a quick hot bath out in the wilderness on an RV generator.

This is a submersible and water proof bath tub/swimming pool water heater and it seems to be made durable and price isn't bad.
I admit I was a little worried about sticking anything electric into the bath tub with me. But, it was the best bath ever when I finally did use this to keep my water hot and I didn't have to keep turning hot water on with this product.
It is completely waterproof and poses no threat obviously. Of course, it is wise to always check the device for broken pieces or anything like that.
I have not used it in the pool yet but, I'm very certain that it would have no problem warming up the water at all after a little time.
It seems to be made durable with safety in mind. I recommend this whole heartedly.

I got this for our backyard pool and was so skeptical about this I was afraid it would melt it or something but it works so well. It is small but it can heat up my pretty big pool quickly and keep the water warm so it is always perfect for swimming. I love this.

This is an immersion water heater. You put it in water that you want heated enough to be comfortable to swim our bathe in. We got this so that we could use it in our small inflatable pool. (It states that it will heat up a 200 gallon pool, or 30 gallons in a bathtub.) When the weather in the fall turns, we would like our pool to be comfortable. This water heater is absolutely the best! It is made 100% waterproof, everything on it is heavy duty. You must operate this fully immersed or submerged in the water, otherwise it will have problems. Other than that, they really are no rules, other than plug it in and when you're ready, unplug it first and remove it. It also could be very handy if you ever needed to heat up smaller batches of water, especially if for some reason your water heater is broken, or maybe you were out camping and wanted warm water to wash in. Our secondary use for this is going to be to warm up the dog's bath when we want to wash them outside. This water heater has a 118-inch long cord (which is over 9 ft long). It also has it's FC certification, which is important! This water heater, being 3,000 watts, is much stronger than all the others. It really kicks out some heat in the water. I am very happy to have this water heater, it is an excellent product!
NOTE: YOU MIGHT NEED A T BLADE ADAPTER, the plug has sideways fins.