Not sure of the true legalities of this, but I've heard suggestions that an AirBNB style "letting" isn't the same as renting, at least with a not-floating home. I have no idea how CRT view casual, paid "use by friends" type of letting as opposed to actual "renting".
I also know of one situation where a boat owner allows a friend to live on a boat as "caretaker". They have a signed agreement and the friend pays all costs, ie: mooring, licence, insurance but no actual profit is (officially at least) paid or earned. Something like this can work where one owns a surplus boat that they don't want to necessarily part with.