NASA worked with a commercial partner to develop the SUNFISH [1][2] for future exploration of Europa. It's been used to explore underwater caves and they have a video showing the generated data from Peacock Springs [3]

[1] https://oceanexplorer.noaa.gov/explorations/22sunfish/featur...

[2] https://sunfishinc.com/

[3] https://www.youtube.com/watch?v=mpKFkrUeF9o

will probably never get those commas, hopefully wont need them because this video by Polish students made one of my 'retirement projects' "underwater ROV"

https://www.youtube.com/watch?v=P2kChvtPxyw

they're hurrying along though https://www.youtube.com/watch?v=VZucylrwaK0

There already is a company doing this, I’ve heard the CEO speak at a conference. They are heavily involved in the cave diving community and will likely start making long distance mapping records in the next few years.

https://stoneaerospace.com/

A submarine drone could probably be done as a hobby project with a fairly limited budget.