For what it is worth, I once had to shoot interior construction damage photo's in a water filled railroad tunnel. The water was eight feet or so deep, I placed my Davis and Sanford tripod legs into 10 foot sections of electrical conduit. I Paddled a rubber one man raft to the spot, then set up my rig. The regular head worked normally but only supported a B&J 5x7 camera. The tunnel was standard guage, so the make shift tripod legs worked by standing on a nearly flat grade under the water. The conduit was about an inch and three quarters I.D. as I remember and good old 100 mph tape held the two together. I used a simular rig with a ladder at one time to do some telephoto images for the Denver Bronco's. Used 35mm for this however.