@NickGri made a comment:
Can we come up with a reasonable, low-cost ROV concept, built of ‘Ferrocement’ (materials substitutions allowed, such as Basalt Rebar and Geopolymer), to inspect and at least minimally service Seasteads, as well as such extant additions, say kelp beds, Aquaculture Fish Cages, etc.?
It would need LED lighting, low-power/high-def video cameras, a waterproof rotary scrubber for biofouling, possibly means to repair fish pens, etc.
Buoyancy control could be by means of a variable displacement system, such as those employed by rc submarines. One such design uses a mechanical piston to vary water ballast, w/o using additional stored air, increasing internal volume requirements, but eliminating the need to control high pressure air.
The drive system would preferably be water jets, rather than external props that might become fouled.