ANSWER: This depends a lot on size, as well as materials and use. Let's first consider the material is either plywood or of a foam or balsa composite.
From personal experience both with well-sealed sailboards as well as the typical plywood ama, I can tell you that they need to breath or they will ultimately suck in moisture. One may ask, how come?
Well the air inside will heat up from exposure to the sun and this expands the ama and adds significant internal pressure. Sooner or later, a pin hole will be found to relieve that pressure. Then, when the ama is put in the much cooler water, the air inside contracts and if that pinhole goes under water, it will suck water in and then, cannot escape. I have seen supposedly sealed sailboards become quite waterlogged through this repeated action and double in weight.
My preferred solution is to add some form of vent that will also be quite watertight when closed. Sailboards are sometimes fitted with a 'pigs-tail squiggle' of tubing that allows the board to breath but is twisted and long enough to not take in water should the board flip over and immerse the open end.
For small amas, I would recommend the installation of a small plastic valve that can be manually opened while the ama is ashore or needing to adjust to a new temperature, but can be securely closed when at risk of being immersed. Such valves are available from common sources and here are two. For a small ama, one can cut off the threaded part of a small water bottle top and bond this over a hole in a deck or bulkhead. For a larger ama—say 10' or more—one can bond on the spiral valve from a 5-litre box of wine. The useful inner part (black) can be punched out of the ribbed (white) part with a large dowel (see pic below) and then the threaded part cut off so that the flange is accessible for bonding to a bulkhead. (Bond it inside to reduce the projection.) I would use the same solution for venting closed end-tanks on even a kayak. These spiral valves from wine boxes are both tough and very watertight.
Keeping in mind that sooner or later, every surface on a boat will be exposed to water and moisture, ALL surfaces inside tanks or under decks etc, must be coated with something water resistant. For lightweight, ease of application, good performance and low cost, I am a big fan of using simple, heat-resisting aluminum paint. Tests I've conducted with this very thin (kerosene-like) paint has shown very good surface penetration and rot resistance. The only negative is that all epoxy work must be completed first, so one needs to plan ahead.
Many plywood amas have been built without adequate interior protection—particularly under the decks. Then, as the ama is commonly stored upside down, any moisture in the ama drains to the unprotected deck and soon rots it out.
During construction, I therefore prefer to pre-coat any plywood that will form the deck with 1 or 2 coats of epoxy. Adding a thin cloth at the same time will provide a thicker layer as well as adding important stiffening to the deck underside that will be in tension from any outside pressure or weight. A little coating weight added early will prevent significantly more weight (plus rot and repair issues), being added later.
Larger amas (20'+) will usually have a hatch in the deck to take advantage of the space for light storage. Of course, water will then also find its way through this hatch so all internal surfaces must be properly coated to deal with this. I recommend that as a minimum, an internal strip of glass be laid in the bottom to better resist a small amount of 'bilge water' that could stand there for a few days, although regular sponging out is highly recommended.
Coal-tar epoxy makes a good overcoat seal for this. All the upper interior surfaces could be coated with 2 coats of aluminum paint, though as noted earlier, I prefer to first add a glass cloth to a ply deck underside, as much for stiffness and strength as for water resistance. Clearly, this is best done by precoating the plywood underside, before laying it.
For hulls made of polyethylene or solid fibreglass, there is certainly far more natural resistance to water though regular sponging out is always recommended. This is particularly true in areas where water or moisture might freeze, as ice expands with great force and will delaminate or damage any area that holds water in confinement. For these materials, adding vents will still lower the risk of hull deformation, though.
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