I shoot a lot underwater BUT digital. Have test before long long time 6x7 with a Mamiya RZ II and it was posible but not that easy. 4x5 or 8x10 underwater would be cool but practically it would be hell especially if you shoot people. I have with digital a hard time to shoot fine art so i imaging those big cameras would be more as a submarin:-)
But yeah a video we need to see if you ever shoot 4x5
My UW photos are on my webpage www.apneaimages.com under Mermaids, Diving & Aqua babies for those that like to check.