T&T: Troubles with a GFCI shorepower plug

[Steven Dubnoff]

I just left my boat for some repairs at my favorite B.C. 
woodworkers.  Their dock, like some others in B.C., are wired with 
standard 110 outlets.  I plugged into the thing with a 30 amp to 15 
amp adapter.  The bad news is that the outlet had GFCI.  There was no 
way I could plug in my boat without the thing tripping.  I even 
turned off every breaker on my panel.  As soon as I turned the 
double-pole (110 volt) main breaker on, the dock outlet thing would 
trip.  So it appears that the problem is not  not in any individual 
circuit, but somehow after the main breaker.

Does anyone have any idea what is  going on here?

Thanks,

Steve