It's a interesting one, and I don't profess to know the answer, but I would have thought the following points relevant in getting closer to it:
1. How long does your client spend in the UK each time they come over to work?
2. How long do they spend it total each year?
3. Do they spend any time working on their boat? (I'm guessing little to none if they are a carer).
4. I'm guessing the boat is replacing hotel accommodation or a short term let rather than the actual travel itself?, which leads me onto
5. Do they let the boat out when they are not in the UK and receive an income from that?
6. Are they employed by the people they care for or an agency or are they self-employed or through their own company?
7. If applicable, given the above considertaions, how long does each care contract last?