Can you sell your boat with the remainder your mooring (anual contract that is), if your boat is in a region where there is a waiting list this could be an advantage
You need to look at your contract. In France you probably wouldn't have the automatic right to do so but with personal contact they would try to accommodate you. Where they don't, some people have sold a half share in the boat one year and the remaining half the following year. This way a de facto occupation exists. However all contracts are annual and so there is again no automatic right.
I think if you look at your berthing contract you will find you do not have the right to sub-let your mooring (even though you may not be taking cash for it, effectively you are entering into a contract with your purchaser). Having said this, I've done it myself and simply let the marina staff know that somebody else had bought my boat and would be using the berth for a while and never had a problem