Unfortunately water damage is not covered by any warranty, however, try placing the iPhone in a zip lock bag of dried rice for approximately 5 days, resist the urge to turn to it back on. There is no guarantee and it maybe beyond saving but worth a try.
First, turn your phone off. Then, you can "try" putting your phone in a ziploc bag of dried rice for 3 ~ 5 days. If that doesn't work, Apple does have an "Out of warranty" replacement program, cost for an iPhone 5 is US $229. Make an appointment at any Apple store.