natural spots in tokyo - water, rocks?

I'm looking for a natural area in tokyo, with a wide expanse of shallow water and rocks. Perhaps a stream, without many people nearby. It's for a photoshoot.

Does anyone know of good spots? Ideally within walking distance from a train station?

Or even a nice beach without many people would be good.

One of the most open places where you can enter the water is the Tama River. Go to Futako-tamagawa Station and you will be next to the river. On either side are wide banks and areas for picnics, bicycling, etc. I have seen many people both fishing and swimming in this area.