Hmmm. I think I understand vintner's problem - you don't want to tread on too many toes in such places - 'cos YOU have to moor there unprotected afterwards.
I'd get someone else local to break the news to the guy first - maybe pop a nice wee note on the boat too - saying you're gonna need the mooring (next week) and while he's welcome to use it until then, you need it back.
Go down at 3am, fin out, cut it loose and make sure the bung is out. A good spring tide in the "right" direction helps too...
rural diplomacy, i has it