Q:why do aviators have ultra-cheap fuel for pleasure then?

A: Because their representative body fought to keep the price down to the EU minimum unlike the RYA who adopted the views of a few Sailing Boat members who expressed concerns about the environment. They chose not to fight to keep the tax down to the EU minimum.
It is not too late to get the government to adopt a tax of 21ppl. If you do not want a 600% Tax increase you need to write to your MP , the Chancellor at DarlingA@parliament.uk and the RYA (Tell RYA to get behind this campaign now and work for benefit of all boating members)