Yes but the taxes here in the UK are so high that they are way out of proportion to the cost of fuel. You are talking about a four fold increase in fuel prices if it went up to the same tax level as road fuel here. That would price many motorboats totally out of the hobby, as well as wiping thousands off the value of their boats.
I expect most users would not mind a sensible mild increase in duty, but you must remember that we have the most expensive road fuel in Europe by some margin.

Steve.