沿海 (yán hǎi) refers to the coastal area or region, often used as a modifier (e.g. 沿海城市 'coastal city'). It contrasts with 海岸 (hǎi àn), which specifically means the coastline or seashore, and with 海滨 (hǎi bīn), which emphasizes the seaside as a place for leisure. 沿岸 (yán àn) is broader, covering both coastal and riverbank areas.