Can’t speak for Ed, but I’m going to guess (unless he’s planning a remake of “Hunt for Red October”) he’s looking for an active sonar. 
Harold’s right, sonar for ROV’s aren’t cheap. A Tritech Micron (sector scanning sonar) will run about $15,000 and a BlueView P90 (imaging sonar) will run you about $50,000. Imaginex also makes a sector scanning sonar that costs a bit less than the Micron, but not by much.
There are a couple of consumer sonars that might be worth looking at. Humminbird offers a “360 Scan” sonar which paints a 360 degree scanning image. Range and resolution aren’t that great but cost is around $2000-3000. Another is Garmin’s Panoptix, which is a forward scanning sonar and runs around the same cost. I don’t know of anyone who has tried repurposing either of these for ROV work, but a couple of us have talked about it.