Hatches like the ones that I built to seal the amas of our trimaran
might fill the bill. These have remained totally dry for 15 years now, even though they are occasionally submersed by deck
They require 3/8" gaskets, (10 year life span), which are really cheap
and easy to replace. And we can only dog them if standing on that corner of the hatch
. This keeps gasket
contact tight, and keeps the dog from moving.
To dog from the outside, we used micarta turnpegs with an adjustable SS machine screw for their axle. The SS machine screw threads into an epoxy plug
in the hatches breakwater, which I later tapped threads into.
In your application, the problem would be that no one wants to sit on the turnpeg.
If your hatch
comes to the edge of the seat, standard hasp will provide the same 100 + pounds (each) of downward pressure. They provide the lock too.
Ours can be locked by running a long pad lock through the three SS eye screws in the middle of the hatches opening edge.
There are lots of ways to do this. If all of the external hardware
is too problematic, You can use a hole saw and install "recessed", flush fitting 4" Beckson deck
plates (with a "o" ring seal), into the hatch top or footwell face. You could unscrew the plates, reach in, and tighten or release conventional hatch dogs
In this case, you would not need a lock, as it is not obvious how to open the hatch!
Hope this helps,