Hatch that Doesn't Need to be Opened Often
You could make a removable cockpit section- what FRP boatbuilders call a, "soft patch,"--by landing the hatch on a perimeter " shelf" on to which a gasket is applied and through which the screws run. Then, the caulking of your choice over top. It's pretty common for fuel tanks in glass powerboats to have this kind of access. The technique is well- proven.
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There are two kinds of boaters: those who have run aground, and those who lie about it.