So you're in Niigata and need to cool off? Head to the beach! Beaches run the length of Niigata City from just west of the mouth of the Shinano River to the mouth of the Sekiya diversion channel, and then from there to the mouth of the Shinkawa. There are several official swimming areas with restaurants and beach houses between the Shinano River Mouth and the Terao Nishi area, but after Terao Nishi there is functionally no development at all, just the sand and the surf. It is this westernmost stretch of beach, known locally as Ikarashi Beach, that I recommend most highly. It sports white/gray sand and clear-ish green water, 10 meter high sand dunes green with dune grass, a purple dune flower (the name of which I don’t know) and a forest of black pine trees between the beach and civilization. As Japanese beaches go, Ikarashi beach is pretty clean. I’ve even seen a group of dedicated locals picking up trash and taking it away.
Go surfing when the waves are up, go swimming whenever it's hot enough, have a bbq, set off fireworks, build a bonfire, take in some sun or just enjoy the beach in your own way. Whatever you do make sure you stay to watch the sun turn red and disappear as it sets behind Sado Island. That's a site you'll not soon forget. From about the end of June through August (and maybe afterward as well, jellyfish dependent) the water is warm enough to swim in after dark. A moonlight swim with bioluminescent plankton sparking blue and white all around one’s body was not an experience I’d ever thought to have until I did one full moon in July.
Be aware that Ikarashi Beach is not an official swimming area (kaisuiyokujo 海水浴所). You will be responsible for your own safety and the safety of those around you. The water is, however, shallow enough to touch bottom almost 100m out, and with currents functionally non-existent on calm days it is pretty safe.
Ikarashi Beach, and all other beaches mentioned here are easily accessible by car or bike from National Route 402 (the Yuhi Line), or by a 15-30 minute walk from any train station on the Echigo Line between Sekiya and Uchino Stations. Get off at Terao, Niigata Daigaku Mae, or Uchino Stations to access the stretch of beach I recommend. Happy travels!