[ryan71]

Boat engines are not typically operated as often as car engines. Perhaps the left over ethanol fuel in the boat engine sits for extended periods of time and hardens like cholesterol in your arteries?

Over time the build up becomes greater and greater until it restricts fuel flow, injectors, and coolant flow.

[Pelham]

Ethanol tends to evaporate from a carburetor when it sits for an extended time.