I don't think insurance companies are interested in whether or not you live on a boat I think there more interested in if you have a permanent address where the car lives most of the time when it is not being used . If the marina is an official residential mooring then it shouldn't be a problem to move your official address to the marina.
Technically if you are 'living' on your boat on a holiday mooring then officially that is not where you live. You live wherever your official address is registered technically. As you said if you lived in Bedfordshire in bricks and mortar then there isn't a problem. Technically you do live in bricks and mortar. How can the insurance company prove you don't 'live' at the official address very often?
I've just read the detailed terms and conditions of my own insurance policy (Admiral) and nowhere in it does say that you have to live in a house. All there interested in is my official address and that I should notify them if my official address changes, or the car is kept somewhere else overnight.
A the end of the day insurance is a contract. Being too truthful sometimes works against you. As long as you give them all the information they need and ask for and you don't contravene the terms and conditions set out in the small print then you have nothing to worry about.