A recent survey of coastal berths in the UK estimated availability at 150,000, split 41 per cent pontoon berths, 28 per cent swinging (buoy) moorings and 31 per cent other types such as fore-and-aft (between piles) moorings and dry moorings.

Further info here

I haven't seen figures for inland yet but that will add something well into five figures.

You would certainly be safer assuming a relatively high average value for the 41 per cent of 150,000 moored in marinas around the coast.

If you want to estimate the true value of the industry you have to consider the value of reliant services and related activity. Plus of course the value to the UK of inward boat tourism -- I think this is one area that would get hard hit by a three-times increase in duty.