I guess maximum 2 to 3 inches, most of the time it goes in because of soft beach sand or mud.
Exactly about the salt spray over the whole setup. Still gotta clean the camera and lens in addition to the tripod, so the amount of post maintenance work makes me think about doing that to cut down the post work to get those sand out. If so, only a wipe down is needed on the tripod.
It would also make me less hesitant about trying more wet shots when I know its ok to get wet since the tripod is wearing Wellingtons.