Moisture can become trapped within the plastic layers and touch pad and related circuit membranes of that are contained in many handheld electronic devices. Water can persist in this form trapped and virtually sealed between these layers for far longer than you might imagine, and cause corrosion and other issues.
Speedy drying is generally the key here, and many people have brought cell phones back to life that were fully immersed in fresh water for several minutes. You immediately take the battery out, open up the back and blast it with a fan or (very carefully) with a hair dryer set on low. It’s surprising how often the phone can make a complete recovery once the moisture is dried out.
If not removed once the moisture has had time to set in and corrode contacts over days or weeks (per Cell Guy’s example) the possibility of full recovery is much less probable. Opening up your phone will allow much more through drying, and the ability to see if the damage is already done.