Yes, I know the link says one claim only. But how can you police that? Two or more people living on a boat each claim. Why not? There's no workable ID. They can't check against people who have already had it, because they don't know who has as it was paid via a reduction in bills, not to individuals. Anyone with a boat could claim. Or who says they've got one.
Presumably the government wonks have been working on this for months and obviously haven't yet found a solution that limits it to one liveaboard per boat. Bearing in mind they'll know nothing about the difference between boaters I'll be surprised if they invent anything rational. They've struggled to find a solution for static parks and aren't even sure that will work. It's an open gate for fraud and while the government appears to have no problem with that at cabinet level, it doesn't like it lower down the food chain!
The idea that you claim, get rejected and then claim for the councils emergency fund is, I suspect, a nonstarter. The council, if it has any money left in the fund, which even the NBTA admit is unlikely, will want to give it to their residents, not noncontributing itinerants, or as has been pointed out, technically illegal leisure mooring or marina dwellers who they will regard as CT dodgers. Most councils can't even afford to fulfill their legal obligations, let alone hand out money to those for whom they have no responsibility.