Water has impurities in it. Some of them probably corroded the contacts of the on-off button. It's too late to prevent that now, so all you can do is have the button replaced. Next time, IMMEDIATELY remove the battery, then wash the phone/tablet in isopropyl alcohol (rubbing alcohol, the stuff you get at the pharmacy). Soak it, swish it around, change the alcohol a couple of times. (Don't worry about getting it "wet" - alcohol is hygroscopic, it absorbs water, getting it out of the phone.) THEN dry it out in uncooked rice for about a week.
It's not the water that causes damage (unless the device is running while it's wet), it's all the junk in the water that does. (And that "pure" water you get from the filter you have on your tap has enough impurities per teaspoon to destroy hundreds of cellphones.)