The coast around Alibaug is essentially one long beach broken by small headlands and the rivermouths that run down from the Sahyadri foothills. Most are usable for paddling rather than serious swimming — the water is shallow, the sand muddy, and the surf gentle.
The main town beach is the busiest. Heading north, you reach Varsoli and then Kihim — both quieter, with better sand. South of town are Akshi, Nagaon, Kashid and Murud, each progressively more remote. Pick by mood — town beach for vendors and atmosphere, the outliers for peace.