South Beach (also called Katama Beach) is the longest and most-used beach on Martha's Vineyard — three miles of soft white sand backed by dunes, with regular Atlantic surf, occasional rip currents, and a bus service from downtown Edgartown in season.
It is wide enough that even on a peak July weekend you can find a stretch of sand to yourself by walking ten minutes east of the parking lot. Lifeguards are stationed at the main entrance only. Bring water, sunscreen, and an umbrella; there is little shade.