Ground fault device trips.
The marina may be fine and the problem may be on your boat (AC circuit).
One of onboard AC devices has a short from the device to its chasis.
This is why it does not trip on the genset - you are not using the marinas' ground wire BUT you still have the issue onboard.
I had the same problem on one of the boats I worked for.
Try the following:
DISCONNECT ALL AC equipment
from the boat AC wiring. The trick is to NOT SWITCH IT OFF but to DISCONNECT COMPLETELY all AC stuff (microwave, washer, dishwasher, water
heater, stereo, step-down transformers ... plenty, in most boats.)
Now try again with the shore power
. If it trips, my lead was wrong.
If it does not, connect the equipment
one piece at a time and find the faulty one.