海洋 (hǎiyáng) means 'ocean' — the world's largest, continuous bodies of salt water. 海 (hǎi) is broader: it can mean 'sea' (a smaller part of an ocean), 'ocean', or be used metaphorically (a sea of people). For the global ocean concept or in formal writing, use 海洋; 海 is the everyday word for seaside or a particular sea.