I don't honestly know without checking against BSS regs. They only show ratings for A and E on the side of the extinguisher so you may have problems with getting a BSS inspector to sign off if you only have water mist on board.
The good news is that they can be used on A,B,C,F and electrical fires (anything you're likely to encounter on a boat) to great effect. The only type of fires that they're not recommend for are type D which are metal fires. Unless your boat is built of Magnesium, you should be OK with that.
I was talking to the guy who sells them to us (an ex fireman) and he said that the problem with getting them rated on all fires was to do with the way the testing regime works rather than with the extinguishers themselves. I can't remember the details but it was something to do with having to design a new test to cope with testing water instead of CO2 or Powder. OK, he was selling, but he looked me in the eye and I believed him. Anyway, you can see from the video how well it works on liquid fires.
I'd carry the minimum required for BSS and a couple of these on top. I've got two 1L water mist extinguishers next to my desk as I type this They're light and handy - just what you need in a tight space.
here's a better link
http://www.safelincs.co.uk/e-series-water-mist-fire-extinguishers/