I don't know about that. If the boat is moving and you're left stranded then yes, of course. If you fall off a stationary boat in warm weather and climb back on though, your destroyed phone might be at the top of the list of concerns. I can see why someone wouldn't want to carry around a non-waterproof phone on a boat. Even if the phone is waterproof (resistant), a zip-up pocket is probably a good idea. Water resistance wouldn't help you much if it's at the bottom of the ocean.