Another option is a Beckson pry-out hatch, which might be a bit easier to use at sea. Betsie Bay Kayaks has use them on their production stitch&glue kayaks as standard-issue for a number of years. They're about the same size ~6". There's a large O-ring around the hatch lid, and you just push the lid into the bezel. BBK installs a small knob on the edge to make removal easier -- just pull up. I think you can find these at West Marine. One drawback is that they're white, but BBK just recently started mounting them under the deck with the cut-out epoxied to the lid for a nice, flush look.

Dean