That sounds like a "slipway", Nemo, a term I do know. It doesn't sound like a place to moor a boat.
On US sites I find "slip" (and "boatslip") described as a mooring place for a single boat. I couldn't find "slip" in a UK glossary of nautical terms, and Collins, a UK dictionary, tells me that "boatslip" is an American word. So, I'm thinking "slip", used in this sense, is too.
In which case, I'm pleased I have a good excuse for not having known it
