There are way too many things that could have happened given that it got hot and then placed in a cold, moist environment. Best case is that the battery was toasted and simply needs to be replaced, but it's really hard to know if can't be powered on. I'm pretty sure if you took it into an Apple Store they would want to run a full hardware diagnostic, but I don't believe that works if the battery has completely failed.
Worst case (as long as it doesn't look like it's been run over by a car) is that they trade it in for a replacement device at the out of warranty replace price. Here are the US prices:
iPhone Repair - Official Apple Support