大海(dà hǎi) - the sea, the ocean in Mandarin

大海 (dà hǎi) is the sea, taken as a whole. It is what you look out at from a beach and what a ship sails into. Despite the 大, it does not mean a big sea in contrast to a small one: the pairing has fused into the ordinary way of naming the open water.

It behaves as a definite mass noun, so it usually appears without any measure word or article: 看见大海, 走向大海, 大海很平静. The set phrase 石沉大海 (a stone sinking into the sea) describes a message that gets no reply.

The trap is the split with plain 海. Named seas drop the 大 entirely, so it is 地中海 (the Mediterranean) and 黄海 (the Yellow Sea), never 大海 with a name attached. Compounds also take bare 海: 海边 (seaside), 海水 (seawater), 海鱼 (sea fish). Reserve 大海 for the sea in the abstract, and use 海洋 when you want the scientific term ocean.