Within walking distance is, of course, debatable. You can call Cottage by the Water With Boat Dock. (Or Google it) very good reviews. Many say "close" to Sponge Docks. They have bikes you can ride to the docks. No hotels within walking distance that I know of.
Hampton Inn, Fairfield inn personally I never stood in any of them, I live close to sponge docks. But you can always look online and they have good options plus check airbnb.
They are a lot of great Airbnb's in the area, that would be my suggestion.
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