I rang two insurers - Ford Direct and Admiral - last week with a view to understanding whether they'd offer me cover as a liveaboard based in a marina. The only reason I picked those two out is due to them coming out cheapest on a comparison site. I was absolutely honest about the potential situation, that I'd be living in a marina, on a boat, and that the address on my policy would not be a place of permanent residence (be it BoatMail or a family member, I offered both), and the car would be kept at an address different to that to which the policy is registered, a marina in my case.
Neither had a problem with this setup after being placed on hold a few times whilst they checked with the underwriters whether it was acceptable. I think it may have been Ford which said that I'd have to put down the marina as the policy address - I explained I wasn't keen to do this as they don't handle mail on my behalf and it's also not residential - but they helped by suggesting that if I opt for email correspondence then no mail need go to the marina. Again, they understood that I did not live in a house. I cannot remember which of each or whether it was both that said I'd have to put the specific pontoon/berth reference as part of the address.
Esure, my current insurer, were clear with me in saying that I have to reside at the address to which the policy is registered, so BoatMail or a family member is not acceptable, even if you're clear in saying the car would be parked at an alternative address. They advised if I were to do this they could no longer cover me and I'd need to cancel the policy. Note, this is important; don't get yourself into a situation where insurance has been refused/cancelled as it's something you need to declare in the future and as I understand it, counts against you. Cancel it yourself once you've got an alternative sorted.
I'm sure others are sympathetic and understanding to the situation of being based at a marina though not sure how constant cruisers might fair.
Hope that helps.