If you look at
bikehike.co.uk - Course Creator you can see our route from Dunwich to Wivenhoe on Tuesday includes a bit from Harwich. There's a ferry that runs at weekends (I forgot this last bit when we went on Tues, expecting to catch it) so we caught a train home from Wivenhoe.
Auntie Helen had a good route through Harwich directly to the North Sea Ferry (different bit from the foot passenger ferry) if you find our German Tour thread of late May / early June.
bikehike.co.uk - Course Creator is a pleasant route from here to Rowhedge which you could shorten by using the Prittlewell Chase / Daws Heath route to Rayleigh and missing out the off-road bit near Langford.
Do check that the Wivenhoe ferry is running otherwise you will have to go into Colchester which is a bit irksome rather than really nasty. They don't run every weekend because they are tide dependent, it seems: if low water is slap bang in the middle of the day they don't seem to bother with the early morning / late evening bits either.