There is a caveat to the longshore/state workers' compensation coverage issue however. I speak of NC workers' compensation law here, but it is similar in other states. Only employers having three or more employees in NC have to have wc coverage, so if you have a guy operating as a sole proprietor working on your boat (your diver or the guy working out of his van) you could now be uncovered for longshore and wc and the worker could sue you for negligence if injured on your boat. Note that if the sole proprietor (owner) has two guys working for him, the owner does not count as one of the three, so the workers still could be uninsured.
In NC, for a premises owner to be liable (negligent) there has to be a latent danger of which the owner was aware, but did not warn about (such as the decks are rotten, but look fine on the outside and the worker falls through and is injured). Open and obvious dangers equally known to the worker (the decks were wet, the worker knew it, but slipped and was injured) are not actionable. This may be different in other states, as NC does not have comparative fault, but is still one of the few with strict contributory negligence.
All of this is to tell you that if you are having work done on your boat by hired help, do think that they will be covered by wc insurance. If it is an individual or small company working on your boat, there is a good chance of no wc coverage and you can be sued if
they think they can prove negligence. It is not out of line to ask to see an insurance certificate for the people you hire.