Only at the Yacht Stations, as far as I’m aware. Don’t think the boat yards would reserve you a place, though it’s always worth phoning up to ask if they have space. I might be wrong about that though. Staying in a boatyard didn’t used to have much appeal to me, until I stayed in NDB’s Yard and found out that it was a lovely peaceful place to be overnight.
There are several pubs that will reserve mooring spots, including The Ferry at Surlingham.