If your depth is 6 feet (not 6 inches) then you may have difficulty staying afloat in places like Brightlingsea (Essex)- deep moorings for bigger boats like golddust as they are 'reasonable', try talking to Morgan Marine (01206 302008) I have found them very helpful
River Orwell has a couple of marinas (Woolverstone run by MDL & Suffolk Yacht Harbour) with all tide access but you pay the price! I am leaving Woolverstone this April for Brightlingsea to halve my annual spend! I am taking a fore and aft mooring which with 3'7" draft will ground at springs!
Titchmarsh Marina in Walton Backwaters (Essex really!) is quite accessable (about 1 metre on entrance to backwaters) and there are buoys as well for a less expensive option.
I don't know the River Deben for permanent moorings but again depth / tides will restrict your access to the river
jana. Suffolk coastal moorings are a rare breed, the places you could try are Felixstowe Ferry Boatyard at the entrance to to R. Deben (you will have the bar to contend with or further up river Waldringfield boatyard both should accomodate 6' draft. Comments re Walton echoed, there are also moorings that are run by Bedwells boatyard, Walton -on- Naze. As already mentioned Brightlingsea is another option on the pontoons but very busy in season with visitors (also long waiting list) speak to HM
Neptune marina is good.. if you are happy with a marina.. access all tides via 24 x 7 manned lock gate. Cheaper than Woolverstone and uffolk Yacht Harbour...
6' draft will be a limitation on the entrance window for both River Deben and River Ore/Alde. That said there are quite a few yachts of this depth that regularly use the rivers, but might be restricted to say at least 1.5/2 hours before high water on neap tides.
Moorings at Felixstowe Ferry and Orford were avaialble last year. Try the FF Boatyard, or Chris Martin at Orford 01394 450267.
Both are excellent locations for deep draft swinging moorings. FF has the added bonus of a reliable watertaxi service if you don't want the hassle of a small dinghy.
Try Jonathan Webb at Pin Mill. He's usually got some good ones on the North side of the river. Loads of water and they don't get picked up by visiotors so frequently.