Ocean pro under deck pic
Wouldn't worry as we have had some water sitting in the void for very long periods. Humber said the void, main channel from upper hull to under deck is sealed. I'd guess 6yrs of testing would have proved otherwise!!!
if you're on the trailer it's probably best to take bungs out to let air circulate and raise jockey wheel up in the air so any rain water sits just in the rear well or drains out of the trunk.
Under way, with bungs out, the amount of water that would get in if you "stuffed" and filled from tube to tube would be relatively minimal... I've done it! :-) Plus it would come out once underway anyway. I guess if you've no auto bilge, water from sports/rain/waves wouldn't be emptied and just slosh about, so bung in at the rear is useful.