Permanently sealed compartments are a favorite discussion topic, especially when combined with foam fill. Possible water intrusion and resultant rot is the boogeyman everyone is running from.
My answer is - depends. If you have a well-sealed stiff structure that is not subject to vibration and flexure and you perform good outside inspections and maintenance, a permanently sealed compartment should be a viable choice. If the compartment has thin walls, vibrates and flexes and depends only on a layer of epoxy for internal waterproofing, access ports are probably a good idea. The right kind of foam in the right kind of compartment will also work well, but you should follow the designer’s recommendations, at least until you have enough experience to understand the results of deviating from them.
Theoretically, a properly sealed compartment will permanently keep out any water. Practically, there is no way to guarantee the longterm integrity of a sealed compartment, hence the recommendations for access ports. However, the absolutely best way to guarantee that a compartment will not stay sealed is to put a hole in it and then plug the hole with a removable plug.
Assuming that you’re sealing the compartment using coats of epoxy, the main thing that will destroy that seal is flexure. While epoxy cures wonderfully hard, it’s relatively brittle on its own. When the wood flexes as a result of vibration, impact or even the repeated thermal expansion and contraction of the air in the compartment, it will start developing micro-cracks. Over time the cracks will expand and join and the barrier to water becomes compromised.
Note that for this to become a problem, the seal on the outside of the boat has to also deteriorate to allow water to get into the wood, from where it will migrate to the closed space.
So if you want to avoid problems with permanently sealed compartments the first thing to do is to reduce flexure to the minimum possible amount. This is easily done by glassing the inside of the compartment. The glass provides tensile strength which compensates for the brittleness of just an epoxy coating. It also resists punctures.
Second, pay close attention to the outside of the compartment. Glass it, too, and make sure to repair any damage as soon as possible so water does not go in through the wood. It’s exactly the same principle as a basement - keep the water outside instead of trying to use sealants on the inside.
Foam fill is mostly useful for preventing water ingress in the event of a catastrophic breach of the compartment. It can also be used for structural support of soles. In any case, if you use foam it should be the Coast Guard approved closed cell structural stuff, not anything sold at DIY stores.
Foam’s big weakness is also flexure. Eventually it will wear down the cellular structure and even cause the foam to separate from the compartment walls. The easy way to fight this problem is the same as for the seal - use internal glass to reduce the flexure.
Another strategy is to be mindful of the compartment’s structure and the conditions that it will be exposed to. A thick-walled robust structure will flex less than a thin-walled one. So you will have better luck with a thick-walled box-like structure under a seat than a thin curved hull section.
Finally, materials count, too. CLC’s plywood cored, epoxy-encapsulated glassed structures are much more robust in terms of water resistance than commercial polyester resin and chopped mat over a balsa core. For starters, polyester resins are porous. Once the gelcoat is scratched there is no water barrier. And balsa core, while wonderfully strong in compression for its weight, is basically a dry sponge.
Hope this helps. For what it’s worth, I have a 23 year-old dinghy with 2 sealed, foam-filled compartments in the bottom and a 20 year-old kayak with an empty sealed airspace in the bow. Both are doing fine. I also have a 4 year-old kayak with vented airspaces that’s also doing fine. The trick is to understand the situation and build accordingly instead of being dogmatic.
Laszlo