When buildung my Spindrift 11N, I did not bother with access ports in my three flotation compartments. I did seal the inside of them carefully with two coats of epoxy. However, this is a garage boat, and condensation is not assumed to play a major part. I will add some 4" round access ports with o-rings in the next few years for air exchange during storage, and they will be added on the vertical sides, not on the deck.
My Spindrift has a decked center section, in the way Garry built it. My big access ports are round, o-ringed, screwed and bedded onto the decks with Sikaflex or similar. One of that section is perfectly dry (the second one has holes for the centerbord retaining elastic rope), and I am very pleased with them. Dry luggage storage makes the difference between contentment and misery when camp-cruising. However, all this adds some weight, and I remember that Garry told me that the same can be achieved with watertight bags.
Axel (Switzerland)