Is your boat a Carburated model? If so, improperly adjusting the throttle cables could cause a too high air to fuel mixture, resulting a lean condition. This situation could quickly cause engine failure. I have seen this many times... (I am a 2 Stroke Snowmobile Drag Race Engine Builder)I had the dealer water test it and adjust the throtle cables.
Also, improper winter storage and a engine that is not "fogged" before storage, can cause a crank to form rust during storage, thus result in engine failure shortly after bringing out of storage... (Fogging an engine protects the internal engine parts from corrosion during storage. This is very important in a 2 stroke motor)