If you installed plywood bulkheads, it's pretty easy to cut some small holes and install a beckson plate (marine stores) for venting. Then you should drill a tiny hole in the plate. If you installed foam bulkheads you can drill and glue in a venting tube. I know that boats that are unvented with rubber hatches really show the pressure in summer. You can see the hatches balooning up. They do gain pressure, plus you trap condensation.