It depends on what you mean by “device”. If you’re talking about a phone the XS Max has the best sound available on an iPhone and if it gets wet or falls in the pool it’ll be fine. If you’re referring to a Bluetooth speaker there are a ton of good ones. Ultimate Ears makes some really nice speakers for the price.
What about some sort of waterproof Bluetooth shower speaker that you could mount on the side of the pool. I did a quick search on Amazon and found the iFox iF012. I have no idea about the sound quality and I'm sure there are others as well that you might want to check out. Good luck and keep us posted!