It seems there are two markets; one for those with money/grants/commercial use, and one for hobbyists like myself with a limited budget. $1000 is the upper limit and $500 would be preferred.
I have been thinking of building a scanning sonar for my (working) ROV. Visibility in the Ottawa River is only a few feet so I would be happy with a range of 10m mainly to avoid running into a rock. Shape imaging would be nice. I will start with one of my transducers from junked sonars. An affordable high frequency transducer which I would mechanically scan would be nice. If small enough and near neutral buoyancy my arm can scan (wave) it. Another option is to take a fish-finder and try to split it with the transducer in the ROV and the display at the surface. I suspect it would be next to impossible. A boat transducer is not designed for depth so another challenge. I use CAT5 cable (4 pairs stranded & tinned) with one pair for RS485 communications, one pair for ROV video, one pair for Robotic Arm video, and the final pair for high voltage charging (converted in the ROV). I would switch the Arm Video pair of wires to sonar video. Specialized custom cables are for the wealthy. Two cables are asking for trouble, even if tied together.
My maximum target depth is about 200m and my present tether is 100m.
Pavel, what form would your video be in?