if you use a nice thick ethernet cable with an isolation separator down the middle, its pretty much waterproof.. not enough gap anywhere to let water in, but it could be forced in with pressure (ie submerged), hence why I add a bit of shrink tape for protection.. I pulled the cover back on my cable, crimped the end, put a few wraps of electrical tape around the crimp to the ethernet wire then pulled the cover up snug over the tape and connector, that sealed off around the cable and cover from the inside.. then the shrink tape was used to keep the twist connection sealed tight and looking nice.
you can see the built in twist cover in this image: