In really rough situations like that I use an old Nikonos, but that's a 35mm camera, and there are only a couple of lenses that are designed for use out of water. The 35mm lens is a beauty, though.
For medium format you could go completely over the top and get a Rolleiflex and one of these:
Probably break the bank account, and possibly your back.
More seriously, though, could you do something with a strong, clear ziploc plastic bag somehow glued to the rim of a UV filter? I think it was once possible to buy these.