Water Street runs along Plymouth Harbor and is the natural starting point for any visit. Plymouth Rock sits at the north end under its portico, Mayflower II is moored at State Pier a short walk south, and the stretch between them is lined with seafood restaurants, casual clam shacks, and gift shops.
The harbor itself is pleasant — working lobster boats alongside recreational vessels, a small beach at the south end. The best seafood option is to pick any of the waterfront restaurants and order whatever the day's catch is: clam chowder, fried clams, and lobster rolls are the standard Plymouth lunch. Nothing here is undiscovered or cheap, but it's good waterfront eating.
The waterfront path is flat and walkable, and connects most of the downtown historic sites within a ten-minute stroll.