Have you thought of suspending your camera upside down in the water instead of submerging the whole tripod? eg using the reversible central column of the tripod, put your camera on, dunk the whole column underwater but with the legs outside (so you must shoot in a corner of some sort).
I think it depends alot on your tripod, some tripods use lubribricating agents that would be washed away by water. A bigger concern is your tripod head. Almost all ballheads WILL rust internally and fail if dunked in water.
how about getting a cheapo tripod and weigh it down with some weights? no need to use a good and stable tripod as the movements in the water are all dampened anyway. free IS everything .. if you are using Canon that is
I am afraid the picture will tun out blur also. The tripod will be very light and unstable due the bouncy of the water.
Tripod mounted on water won't really damaged it. Just make sure the head is not in the water. Dismantle the legs of the tripod after immerse in water to wipe and leave to dry, apply light lubrication on the thread.
*based on Gitzo tripod design*
Mounting the coulmn upside down will never work, unless you want the legs of the tipod to be part of the paranomic image.