Blue Robotics (bluerobotics.com) is also using Pi, for their BlueRov and survey boat (BlueBoat) - they’re quite nice to use (and not toys… you can add sonar, dvl, usbl locator to go at depth - max 600m with the kit)
I’m also starting to build other water/ground vehicles for underground exploration - communication and localisation are challenging, without speaking about navigation on uncertain terrain. Pi are accessible and cheap, and the Pico’s are also very useful to interface with the motors, sensor and so on.
Sadly no — you have toys (none open source and reliability is... well, let's say that the repair are regular and out of support) or you can try to create yourself a waterproof solution (that's complex). Most connector cost a lot, 3D printing is not really an option (especially at depth), pressure is not joke to deal with... And then you'll have to fight with the environment (algae, plants), water salinity, acidity and so on.
Other system at the same level costs more — I think the next one is at 25k without option.
Don't forget that if for any reason you need to reach it at depth, you'll have to dive. Please don't do that alone...
I’m also starting to build other water/ground vehicles for underground exploration - communication and localisation are challenging, without speaking about navigation on uncertain terrain. Pi are accessible and cheap, and the Pico’s are also very useful to interface with the motors, sensor and so on.