In general you need the permission of the harbour authority which is responsible for that bit of water. Outside of their jurisdiction then I presume you can land but probably not do much else if they forbid you coming into port.
I tried to secure permission to operate a sea plane out of Portland Harbour in Dorset. It went back and forth a long time but at the end of the day I could not get permission. In fact back in the 90's they even passed a ruling banning "vessels capable of flight" - after some chap started operating a rib-type microlight from there (looked a lot of fun actually). Trying to get these rulings overturned is like banging your head on a wall.
I thought about Poole Harbour as there used to be seaplanes based there many years ago but by that time I had decided enough was enough and gave up on the idea. Shame really or else I'd be flying around in a nice beaver now !