If the pressure was great enough, it would force fuel out the vent. My guess is the pressure you get at the cap, is enough to compress the airspace, but not enough to force the fuel out the vent.
I wouldn't fill the tank that full, because if it does get hot enough to vent, fuel will puke all over the side of the boat, and on to the ground. If there was an ignition source in the area, it could get ugly (even though, by code, the garage itself should be vented)