I was interested once in mooring between some buoys on the Itchen and they were controled by the Port Authority, unfortunately their contact details and the mans name are now too far buried in the grey matter to be recalled.
I have one available. Walk on walk off, fresh water and electricity, high security and ample free parking. Cost per annum just £700. Sadly its upstream of the Northam bridge so only suitable for power or maybe gunter rigged dayboat.