you might get access from Methersgate Quay but its all private land.
The Tide Mill would be the best bet or Granary Marina further up the river but more restrict due to drying
I'm advised by a mate of mine that you can get ashore from a dinghy opposite the Tide Mill (sort of) and there is then a public footpath up to Sutton Hoo
It's not far at all from the Tidemill or Granary, along the footpath by the river then onto the road and across the bridge, and up the hill. I have cycled it a couple of times but it won't take long on foot either.
It would be about a three-mile walk, along the sea wall to Wilford Bridge (the interesting bit) and then right at the roundabout along the B1083 to the NT Sutton Hoo site drive (the not so nice bit).
A ferry once operated from Woodbridge Quay to the Sutton Hoo shore, hence Ferry Cliff with Ferry Hill above it.
If you could land and leave the dinghy somewhere opposite the Tide Mill yacht harbour below Ferry Cliff, it would only be about a mile walk on the public footpath that runs from the Cliff along the saltings in a NE direction past Sutton Hoo Farm before going uphill towards the Visitor Centre.
OS Explorer Map 197 Ipswich, Felixstowe & Harwich is useful.
Using the dinghy option might involve getting muddy at one end of other of your trip depending on the state of the tide, but the channel does run quite close to the Cliff shore.
However you get there Sutton Hoo is definitely worth a visit, as indeed is Woodbridge if you haven't been before.