Canada

I feel you. I think at a bare minimum anyone wanting to do something like this should be required to get some sort of insurance coverage that will help bear the cost of rescue/recovery if the worst happens.

There's a... I don't want to call it an "easy solution", but there's a pretty understandable and clear distinction that can be made here: If you're offering a commercial service to people, and an investigation finds negligence on your part, the public should be able to sue to recoup at least some of the expenses.

I don't think it's all that slippery a slope. Not many deep sea private submarine rescue operations going on amidst the working class.

I think there needs to be a limit. In (Inland) search and rescue for example, they will do all they can to get you out, but if you're injured they aren't calling an ambulance (unless you desire so) and they don't cover the cost of any kind of healthcare.

In this scenario, it would make sense to restrict or limit aid for non-millitary underwater recovery operations, especially those involving a submarine. The govt should not be bankrolling private companies' experiments IMO unless there is a really good reason for it

Edit: added type of SAR