The actual wording can be found in Excise Notice 554. Paragraphs 5.7 and 5.8 are relevant. Here they are:
5.7 If you’re a residential boat owner and only move your boat short distances occasionally
We recognise that there are boat owners whose primary, or only residence is their boat.
Some of these will be at fixed moorings or move just a very short distance along the tow path from permanent moorings. If you live aboard your craft permanently and hold certain documentation, such as a Houseboat Licence, Residential Mooring Licence, Council Tax bill in respect of the mooring, or other documentation, such as invoices or bills which provide proof of permanent residency, you may purchase all your fuel at the rebated rate (as if you owned a commercial vessel). You are not required to make a declaration.
However, it will be your responsibility to make sure that you hold the requisite documentation should we wish to check this at a future date.
5.8 If you’re a continuous cruiser
If you’re a ‘continuous cruiser’ you may not claim all of your fuel for non-propulsion purposes under the arrangements for residential boat owners. Even if you reside permanently on your craft, you must declare your actual intended usage for propulsion.