the usual test is how much % of the time is the vessel engaged on business work, and how much on personal.

I don't think the Rev 'n C people would be happy to see 90% business without an associated profit and loss statement. Maybe your accountant could advise on how much you might be expected to make from marine photography, and what sort of expenses are fully and wholly justifiable.

I think that somewhere in the regulations, the Customs have powers to seize a vessel if it is using illegal red. That's certainly the case for farm vehicles