The big 8" ports, I think they are standard on the H20, have a reputation for breaking when put under direct pressure from someones foot. The more common 4" ports don't have the same kind of problem, however, you can knock the threads out of whack with pressure and they can be a reall Bitc* to open if you sat on 'em.

West Marine carries a different brand of 4" ports that have much more sturdy threads. However, they aren't compatable with the more common lids.