Amps can easily be underpowered for the load they are seeing. As a matter of fact it is much better to have an amp that is overpowered (truly more power than you need, not just over-rated) than to have an amp that is under powered. That amp will have to work harder to produce the desired SPL and will more than likely start clipping. This is where the waveform flattens out because the amp is not stable at the required current. This causes distrtion and serious heat issues.
Sounds like this could be your problem and it could be as the others have noted directly related to how your 8(?) 4 ohm speakers are wired to the amp.
Also, you are on the right track re: the ground issue and the amp shutting off quickly. If it were a ground problem it would probably be a softer clip, for lack of better terms. However in a boat the ground issue would have nothing to do with whether the boat is on the trailer. It either has a good ground or not.