This dive computer manufacturer, Liquivision, just introduced a dive computer that can communicate underwater using ultrasonic technology. They claim that their battery-powered wrist dive computer can communicate with other divers and the dive boat up to a distance of 330 feet/100 meters.
It seems to me that this technology could have applications for communicating with an ROV and eliminate the physical tether.
Liquivision Lynx Dive Computer
A quick Google search for underwater ultrasonic communications turns up some research and applications in this area.
Whether there is sufficient bandwidth for video transmission would of course be a big consideration.