Water damage, black screen, phone still on! Won't turn off!

Which phone is it? The only thing you can do with a blank screen is try a hard reset (search for how to do that on your phone). Check your warranty for what the conditions are for you to have it repaired or replaced and also for what level of water resistance it actually offers. If you were within the limits it should be no problem. Basically whatever happens it will need to be examined for water damage by a repair technician to find out what parts have been affected. To be quite honest I'd never trust phones + water no matter what the advertising says!