海滩 (hǎi tān) refers specifically to the sandy area along the sea where people sunbathe and swim — a beach. 沙滩 (shā tān) is a broader term for any sandy shore (lake, river, or sea). 岸 (àn) is more general: the shore, bank, or coast of any water body (river, lake, sea) and does not imply sand. 海滨 (hǎi bīn) is the seaside area as a whole (promenade, resort), not just the sand strip.