Actually if the answer is "water got in" I doubt very much that you can save the phone. The advice in that circumstance is to immediately remove the battery, dry it as thoroughly as you can (a couple of days in a bag of uncooked rice is recommended), then just hope. But it's insidious stuff.
If you are able to get it booted again, back up everything you can to sd or cloud. Only real advice I have. HTC won't touch it if it's water damaged.