This Essex based port town has some decent waterside walking trails along the coast and the river on which it sits. The town is also notable as the likely launch point of the Mayflower which carried English Puritans to North America in 1620.
This walk from the town follows a section of the Essex Way to the nature at Wrabness on the River Stour. It's a long walk of about 10 miles (so 20 miles there and back). It does use a series of flat footpaths however and you have the option of catching the train back from Wrabness.
The walk starts next to the pier in Harwich and follows the waymarked trail south past the harbour, Harwich Green and the beach. You continue to Beacon Cliff before turning south west to pass along the coast at Dovercourt Bay. Near South Hall creek you turn inland towards the villages of Little Oakley and Ramsey. You continue north west to meet with the River Stour around Copperas Bay and Copperas Wood nature reserve. You can then follow the footpath west along the river where it will take you to the Oakfield Wood Nature Reserve before arriving at Wrabness.
To continue your walking in the town you could try the Hariwch Maritime Trail which takes you to some of the historic highlights of the area. See the video below for more details of this route.