I didn't see a marina when I was in Lamlash, can't see one in the aerial imagery and I can find no reference to it online (wouldn't a marina have a website etc.?) it looks like it's possible to moor boats here though (https://www.harbourguides.com/harbours/Isle_of_Arran/marinas - "PLEASE NOTE,ALTHOUGH WE ARE UNDER THE MARINA LISTING FOR ARRAN, WE ARE NOT A MARINA WE DO HAVE A NUMBER OF VISITORS MOORINGS. ")

So maybe the mooring tag should be used without adding a marina? Not sure as I'm unfamiliar with how these sort of places are tagged

https://wiki.openstreetmap.org/wiki/Key:mooring