From memory, the Customs and Excise notice (744a?) refers to vessels having 10 seats or more, and it being used commercially, rather than it being defined as a 'commercial vessel'.
The accountant I spoke to compared it to a taxi van - you can buy a 12 seat Land Rover and pay VAT, but operate it as a taxi and you can get zero VAT rated (i.e pay no VAT on the vehicle, and charge no VAT to you customers).
The difference appears a bit pedantic, but I'm going to try and go this route (with the RIB!!) - so I'll let you know how I get on......