Apple will likely charge you for it..
The only way they will be able to tell its water damaged will be from the visible small water sensors located 1 in the headphone jack, and two in the comm port.
These are usually white, but when they come into contact with liquid, they turn red so are instantly visible for easy diagnostic.
Usually, when any retailer sees this, they will turn you away and or charge you for a repair regardless of your now void warranty.
However, If you want to be slightly devious.. you can purchase these little water sensors on ebay for a couple quid..
Quick google on how to get to the sensor in the headphone jack (the comm port ones are easy)... tweasers and tiny bit of glue, you can pull the old red ones off and put the new shiny white ones on.
They wont know the difference until weeks down the line when its all taken apart and there's more red ones inside.. but, by that time its too late because they've already accepted and given you a new one.
Job done