In a salt water environment a conformally coated amplifier is important due to airborn corrosion. In freshwater areas it would have more to due with how you're caring for your boat.
Direct water exposure to any amplifier, conformally coated or not, is often terminal. Getting water in an amplifier while its operational would be certain death. Nothing is totally waterproof.
Ditto to the above comments. Make sure the boat interior is dry before going under cover. Manage drainage of drink holders, overhead vents and other deckcap penetrations into the bilge. Mount amplifiers vertically and spaced off the sole for isolation from the occasional roller. Make sure wiring doesn't drop into an amplifier from overhead without a drip loop. Space the amplifier up off the mounting panel. Things to do.