This section of the moat surrounding the Imperial Palace is one of the city's most popular hanami (cherry blossom viewing) spots. There's a 700m-long pedestrian path (for strolling, not picnicking) alongside the water that's especially pretty when the blossoms are lit up at night.
There's a small pier that rents row boats to take out onto the moat from late March through October (11am-4.30pm Tue-Fri, to 5.30pm Sat & Sun; ¥500 per 30 minutes). During hanami season the hours are longer, from 9am to 8.30pm (and the cost is ¥800 per 30 minutes).