There are trolling motor plugs and receptacles. Might be more what you're looking for. I have also seen a connector about the size of the 7 pin RV connector with only two contacts in it. I've no idea what the application might be or how to start looking for it.
Marinco's locking plug and receptacle combo is a fairly nice system. Can still use std. power point plugs with their receptacle and can also still use their plug in std receptacles, it's only when the two are mated & turned a 1/4 turn that they lock. When used together the plug's rubber seal does engage the bore of the receptacle and should form a seal from the looks of it. I've just not had reason to test this. About the only part of their design that I'm skeptical of is the plastic threads, but I've been using them for several years and I've yet to have a problem.
