What does the phrase As useful as a screen door on a Submarine refer to?

This is a very old joke, and refers to a worthless piece of equipment.

"Screen Door" on a submarine refers to the in port hatch Security Screens, used to keep equipment, people, debris, bugs, etc. from entering the boat by accident (or to slow down a potential threat) while moored in port. If they were in place and the hatch still open, the boat would of course flood. Therefore, the term is a reference for any useless piece of gear.