I’m brand new to this site, and I’m very excited to learn more about what is going on here. I have seen a technique used while I was a student in Boston on the Big Dig project. Essentially, steel half shells were lowered into the bay, and scalloped with other steel shells. This became an inexpensive way to build a sea wall in order to pump the interior space of water.
These caissons could be used, I have no doubt, in 100 ft depth of water. Only, I wouldn’t recommend pumping water out of them, I’d only recommend using them as a long empty wall (with some bracing) that could be filled with, well, trash & garbage. The tops could then easily be sealed with concrete or crushed rock.
What I mean is this. I’d imagine there are many communities which don’t know what to do with their trash. Do they bury it? Do they burn it? Do they pay a community to take it off their hands? If the Seasteading Institute has a large area to fill (such as the interior of a mile long sea wall) and nothing to fill it with, then it may be profitable and legal to allow States to dump their non-radioactive waste in these caissons as fill. Of course, there are also immense floating plastic heaps in the center vertexes of the Pacific, Atlantic and Indian oceans that could also be used as fill, and of course sand from the ocean floor, but these aren’t as profitable.
I’d imagine with a sea wall like this, formed in a circle, then just about any water-tight vessel could safely dock inside.