But seriously there has to be some threshold of common sense. You are sending a sealed box 100s (thousands?) of meters into the ocean (or replace with another similarly risky sounding proposition - trying on experimental jet packs, getting shot out of a cannon with a parachute etc). It can't be that unreasonable to ask "if something went wrong who should be responsible and for how much damages and what level of insurance should be mandatory (or atleast available for sale)" - we already do this for mortgages where risks and impacts are a lot lower.
Now if you have more time and patience sure go ahead and enumerate it and do the actual work of passing regulation around it until that insurance premium actually becomes profitable. The free market actually can help if you ensure it is used.
Even in your example it is true that car accidents are way more. But the number of miles driven in cars is orders and orders and orders of magnitude more then number of miles of ocean floor traversed!
Edit: this also got me thinking a bit more. I can see why in America the R word seems scary and is a bit triggering. Regulation here can be as simple as insure your customers for say a million dollars of no questions asked policy and pass the cost of that to customers and let them (and the actuaries) decide if it is worth it!