You can buy similar off the shelf, the way these diy projects go it may have been cheaper in the long run, but more power to him. It can be hard to drop 5 grand on a kitted out submarine when you think you might be able to do it for 2k in parts in your own labor, but in my experience that 2k in parts starts to creep up as you accidentally destroy things, determine that the things you bought and can't return won't quite work, etc.

There is a guy that has been using one of the off the shelf ones in the lakes around seattle (https://www.youtube.com/@rctestflight/videos), he's also built a bunch of other rc stuff including a few autonomous boats that he takes into the lakes as well as the sound.