Any device that gets wet should have the battery removed (if removable), or powered off if not. Then placed in a bag of rice for at least 24 hours - the longer the better. This is of course assuming the water was just plain water. For Salt water and if they've been fully submerged they should be flushed with clean water - again with NO BATTERY. DON'T FLUSH a device with the battery installed. If the water was just a splash of Salt water, you may get by using 90% alcohol, since alcohol and water mix and the alcohol dilutes the water, washes it away and leaves a quick drying non-residue coating of alcohol.
The real problem with the power button is that it's likely a very small disc of metal (about 3-4mm in diameter), which is dimpled to create a "clicker" button. Then that disc is taped down on the contacts with a piece of (usually) Yellow tape to form the switch. If water gets under that disc, under the tape, it can take quite a long time for the water to dissipate. See below for examples;
(Click on image for large version)
Of course, the information above is a "use at your own risk" recommendation.