Hello Highlander, If it is just a light weight ply or plastic flush fitting hatch then adhesive tape which is one use, mainly used on Class racing yachts.
A flush hatch does need to have something to grasp to lift it off.
I have a scale yacht with flush fitting hatch and use dummy winches to lift it.
The construction of the hatch has 3mm ply edges about 6mm deep and they slot into a U section with bicycle valve tube in the base of the U section together with some vaseline smeered around.
This has proved a reliable seal especially if you can clamp the hatch down in some way.
You could also put a lip under the front edge of the hatch so that slips in place and then have the rear edge clamped down from the deck. I have used cut back servo tops with a screw to pivot on. If painted the same colour as the deck you do not notice them.
If you can adapt a dummy winch to have a machine screw which can lock into a captive nut then this is very effective.
Depend if you are scale or not.
regards
Roy