NOT covered as standard on Home insurance while at home, but some insurers might provide the cover if the policy is a High Net Worth contract (they seem to cover pretty much anything you could think of, but they aren't cheap and only quote if you have large sums insured for General Contents or Works of Art, Jewellery etc). It can be included if you tell your insurer, and they have the facility to provide such cover. Same goes for Caravans by the way.
Best option is to get a proper boat insurance policy. Haven Knox Johnson is a particular specialist in Marine and they offer very good policies for boats. As do Navigators and General (I think they are a subsidiary of Zurich still). Towergate administer the old General Accident's Small Boat and Yacht policy for Norwich Union - not cheap but good covers, if memory serves.
If in doubt speak to a local broker. Most have access to one if not more of the above three, and there are many others out there, but look around and deal through a broker.