I have a very old (late '80s) cathode ray tube depthfinder called a Marinetek Video Sounder.
I have NO CLUE how it works, but it involves a squared-off hull projection with a glass window (I dislike this on general principles...). However, it did work last season and did give a decent and fairly vivid idea of the bottom, but it sucked at depths over 40 feet, and eventually grew a coat of algae that meant it recorded all depths as 3.2 feet...I have to clean it this winter.
Did I mention that the Video Sounder (when scraped clean) gives a pretty accurate speed under way? Over about 1.5 knots, that is. I don't have an impeller thru-hull, just this old thing and a 1990s Raymarine chartplotter that tells me when I'm doing over 2 knots.