You use the phrase "his mooring within the marina" - do you all have specified mooring places, or is it that you do just generally always return to the same space? If you do have specific allocated lengths then you have quite a strong argument, but not otherwise. I'd be very surprised if there is anything to say about access to the canal though - I've not heard of any such clause in mooring agreements used by marinas that I know of.
Are you the only person who does occasionally cruise, or are other moorers equally inconvenienced? I wonder too, why does this guy moor so others can't get in or out? Is it easier or better for him in some way, or is he being deliberately awkward?
These thoughts are interesting, but probably do little to help you resolve the issue. The answers may prompt different forms of advice though.
Tam