John, the river bed in the vacinity to Hullbridge varies from soft mud to hard gravel/stones. I suggest you talk to the mooring officer of the Upriver Yachtclub at Hullbridge.
However, you mention that you want to remain afloat so I would recommend that:
1) you contact Brandy Hole Yacht Club as some of the moorings on the bend in the river should JUST keep you afloat.
2) go further down river to North Fambridge and ask at the boatyard there (by the Ferry Boat Inn) they have lots of deep water moorings and a free watertaxi service.
John, I bought a macwester 26 which had been kept on a drying mooring in hullbridge and she must have been kept on one of the hard stony patches mentioned in another post. the bottoms of the encapsulated bilge keels were very badly damaged and had to be ground right back and re-laminated. Hope this is some use..