I think boat owners can only use duty free red for heating not for propulsion. They are still allowed to run their engines on it, but when filling up they have to sign a declaration saying what proportion will be used for domestic and what will be used for propulsion. They then have to pay the extra duty on the proportion used for propulsion.
I may be wrong, but that is how I understand it.
Marinas in the U.K. still sell red diesel. Commercial vessels can use lower rate red legitimately. As a leisure vessel, you will be charged full rate for the full purchase unless you declare that a percentage is for heating use. Customs & excise will generally accept a 60/40 split as being the default. Liveaboards can declare 100% if the boat never moves. The Marina will log the purchases though, with the boats name and owners details.
you don’t need to claim the duty back, price is adjusted accordingly at the checkout.
The big issue will be when a yacht crosses the channel or the North Sea. Red diesel is now illegal in the EU and if checked, you will be fined regardless of the duty you paid.
I live in The Fens and it’s not unusual to find independent garages that sell reduced price red diesel at the pumps, no questions asked. I collect it in 20lt Jerry cans for use in my tractor. (Off road use only). It would be illegal for me to take the tractor there and fill it!