So a couple of days ago I glorped some silicone grease into an RJ45 connector, then measured the contact resistance when a plug had been inserted. I didn't notice any change in resistance due to the addition of the grease. I had been wondering whether Subconn/Seacon connectors have anything magical in them that allows you to grease the contacts, but it appears that any sort of sliding contact handles that situation.
After I couldn't come up with a good way to mount an RJ45 socket in a way that would be easy to waterproof, my thoughts turned to how to make this connection as simple as possible, and the best I came up with were banana jacks. For instance, Pomona makes a simple mini-banana jack:
It seems like you could press-fit this into a hole in the plastic shell, then add some potting as necessary to make sure the interface is waterproof. Mouser sells these for a buck:
Pomona has a mating gold-plated plug for this:
So the one question I have about these is whether there's enough gap between the tip of the plug and the opening on the jack to force out excess water and grease when you go to mate the two. More experimentation is in order I guess....