I would create a query showing just the available moorings and use that query to feed your combo box.

If a boat is assigned, the boat's name is entered in your table.  If no boat is assigned, the boat name is blank.  So your table should hypothetically look something like this:

MooringName  BoatName
Mooring 1    Boat 5
Mooring 2    Boat 20
Mooring 3
Mooring 4    Boat 152
Mooring 5
Mooring 6    Boat 10
Mooring 7

So your combo box should have a row source that only shows records with blank (null) boat names:

     SELECT MooringName FROM MyTable WHERE IsNull(BoatName)

When a boat is assigned (and entered into the table), the combo box needs to be requeried to show the updated list.