This is Beckson 8" screw in hatch and is very water tight as you can see, and it was a good choice for me. I am thinking of using a small piece of heavy canvas glued over a hole for an automatic breathing pressure vent to equalize the inside to outside pressure on hot days. When the canvas vent is under water it would swell and stop water from interring the compartment, much like a canvas water bucket works when the canvas swells to stop it from leaking.

John