solar cells
Some locations have (soon to be fully AI) birds in the sky, so attempting to move on surface during day beyond the prevailing drift might result in capture, all you could do is sit there surface charging as a piece of "driftwood" while hoping your glass doesn't reflect into the birds. What are current estimates of battery capacity needed per subsea nautical mile? Cryptocurrency powered international distributed shipping, parcel networks, drop gangs, etc will need to know these metrics. However today the latest tech has cut battery needs to a fraction... https://en.wikipedia.org/wiki/Underwater_glider https://en.wikipedia.org/wiki/Autonomous_underwater_vehicle Wave / ocean / sea / hydro gliders, buoyancy engines, etc. They're dual use so you can profit by selling them. It's also highly likely the US has TOP-SECRET littered the ocean with them to complement the SOSUS nets. Maybe Anonymous will score a botkill on twitter, lol.
ghost nets
Try physical spikes and full spectrum sensors to detect before impact, raising odds of success of full reverse thruster. As with 3D-printed guns, the plans can all be opensource, open for all to improve.
plastic trash
Other than surfacing for charge, gliders can operate in safer density zones, outside SOFAR channels, etc.
drugs
https://www.cs.unh.edu/~it666/reading_list/Physical/final_ieee_robot_crime_a... http://www.hsaj.org/?fullarticle=8.1.13 Though as with free speech, cryptocurrency, tor, etc... distributed censorship resistant and free market shipping alternatives among peers will be the legitimate and majority use case. Besides, by that time, crypto will have made any remaining govts abandon their silly futile costly and harmful war on drugs anyway.
interesting hobby
https://en.wikipedia.org/wiki/Personal_Submersibles_Organization https://en.wikipedia.org/wiki/Underwater_acoustic_communication https://en.wikipedia.org/wiki/Communication_with_submarines #DropGangs #DistributedShipping :)