Apple iWatch won't turn on after pool

Start by checking this is not something easy like a discharged battery, and see Apple’s other suggestions in If your Apple Watch won't charge or it won't turn on - Apple Support. If that doesn’t help, your Apple Watch Series 4 is still in warranty cover so Contact Support and arrange getting it checked by Apple.

Two other comments: If the watch gets hot (not just warm) when you try to charge it there is a battery problem, stop charging and Contact Support.

Your watch should be “water resistant” when originally manufactured but damage due to water ingress is not usually covered by warranty. If there is no external damage (cracked screen, etc) there might be no reason to suspect water (without opening the watch which they can’t do in the Apple Store) unless you tell them about swimming. About Apple Watch water resistance - Apple Support

Make sure you have ejected any remaining water by turning up the crown....then try charging it. If it is still non responsive I suggest you make an appointment with an Apple store Genius Bar, if possible......https://www.apple.com/retail/geniusbar/

You have been to the Genius Bar so you already know the answer, that watch is dead. Rice is a waste of time, if it manages to extract any water (unlikely) it will simply concentrate the dissolved content and increase its corrosive effect.

Mine just did this as well yesterday and was on water mode. Swam for no more than 20 minutes. Just brought to Genius Bar and was told that it was water damaged and I would have to pay for a new watch. Absolutely ridiculous that Apple markets their watches to be able to swim with yet won't replace when water damage. Currently sitting in a bowl of rice to see if I can dry it out even more.

Let me know if you have any luck with yours!