Downtown Medford was built with its back to the river, which McGlynn acknowledges was a ‘‘mistake.’’ He envisions boardwalks in west Medford square and water taxis running up and down the river. What he calls ‘‘connectivity’’ between parks on both sides of the river.’’ Today a boat in the Lower Mystic Lake in Arlington can navigate into the Atlantic Ocean. Someday bike and pedestrian paths should go all the way to the sea as well, advocates agree.