I suggest that you reconsider using a screw-in hatch. Any curvature will cause it to jam, as will any sand that gets in the threads. The press-in hatches are more foolproof and if you're concerned, you can easily make small turning latches to prevent it from popping out in the event of a severe hull impact (resulting in a pressure increase in the compartment, thought it's unlikely to be a problem with a day hatch, especially if you vent the bulkheads.

For a tether, just run a stainless machine screw from the outside and attach a ring on the inside for a tether cord.