I have basic electrical knowledge, but don't "get" exactly what you are trying to do. What and how many wires are going into the box? What is the mounting location/situation. Will the cover be plain, or will there be a switch in the box? I'm sure you don't need a marine-grade-waterproof-to-300-feet-depth connector ... especially being low voltage. If you haven't already bought the box, you might be better off with smaller openings instead of 3/4" ... tbd. Your best bet for avoiding water entry into the box is if the wires enter from the bottom. A 2-wire situation will be the easiest.