with the boat in the water, before leaving the dock I turn off the breaker at the pedestal on the dock, unplug the 30amp shore power cord from the boat and the main breaker on the boat panel shuts itself off. When back at the dock, I plug in the shore power cord to the boat, turn on the breaker at the pedestal and turn on the main breaker on the boat.

with the boat on the hard, the 20amp extension cord with the shore power adapter remains plugged into the boat shore power receptacle. I use a receptacle in the picnic shelter as a source for power and plug my extension cord into that. When I'm done for the day, I unplug that extension cord and the main breaker on the boat shuts itself off.

In both cases there is no trip of that main breaker any other time even with heaters and lights on. The secondary breaker on the boat panel for the receptacles has tripped but that was my fault for using lights, heaters and a shop vac at the same time.