Located about 1.5 hours away from Tokyo Station by train, Isshiki Beach is one of my favorite summer getaways. The water may not be crystal-clear, but the waves are calm, the area is not too crowded, and full of friendly people.
After a nice swim or relaxing stroll along the beach, have some delicious food and drinks at Blue Moon (only open in summer) as you watch the sunset.
Isshiki Beach: https://maps.app.goo.gl/LVBA8pLkuMGyhHBh9
Blue Moon: https://maps.app.goo.gl/2wT4kduTLtFDA5p27