海 hǎi is a pictophonetic compound: the water radical 氵 carries the meaning, and 每 (měi) supplies the sound. On the left stands 氵, the three-dot form of 水 shuǐ (water), which marks the great family of liquid and waterway characters: 河 hé (river), 湖 hú (lake), 洋 yáng (ocean). On the right is 每 měi, here doing phonetic duty rather than contributing its everyday sense of "each." The Shuōwén glosses 海 as the great body of water into which the hundred rivers pour: water, plus a phonetic, naming the place where all water gathers.

From the literal sea, three senses fan out:

1. Sea, ocean. The body of salt water itself: 大海 dàhǎi (the sea), 海洋 hǎiyáng (the ocean), 海边 hǎibiān (the seaside), 海岛 hǎidǎo (an island).
2. By the sea, in place and institution names. A city or thing tied to the coast: 上海 Shànghǎi (the city "upon the sea"), 海关 hǎiguān (customs), 海军 hǎijūn (the navy).
3. A vast quantity. Anything as boundless as the ocean: 人海 rénhǎi (a sea of people), 火海 huǒhǎi (a sea of fire), 海量 hǎiliàng (a great capacity, especially for drink).

The thread is vastness that gathers. The sea is where every river ends, the widest thing the early Chinese world knew, so 海 came to stamp the idea of the immense and the all-collecting onto whatever it touched, whether a crowd, a blaze, or a person's tolerance for wine.

Because the sea was the edge of the known world and the door to trade, 海 became the natural marker for places and institutions that face the water. 上海 Shànghǎi, China's largest city, is literally "upon the sea," a name it earned as a fishing and shipping town at the mouth of the Yangtze. The same logic builds the apparatus of a maritime state: 海关 hǎiguān (the customs office, the "sea barrier" through which goods pass), 海军 hǎijūn (the navy, the "sea army"), 海港 hǎigǎng (a seaport), 海运 hǎiyùn (sea freight).

The sea also reaches inland through metaphor. 海外 hǎiwài, "beyond the seas," is the standard word for abroad or overseas, a reminder that for most of Chinese history the wider world was reached by water. To this day 海外华人 means the overseas Chinese, the communities scattered past the coast.

The sea is the largest thing the eye can take in, so 海 became a unit of the immeasurable. Append it to a noun and that noun turns boundless. 人海 rénhǎi is a "sea of people," a crowd so dense it has no edge; 火海 huǒhǎi is a "sea of fire," a blaze that has swallowed everything. The pair 火海 and 刀山 (a "mountain of knives") combine in the vow 上刀山下火海, to climb the knife-mountain and plunge into the fire-sea for someone, meaning to face any danger for them.

The same vastness can measure an inner capacity. 海量 hǎiliàng is "an ocean's measure," a tremendous capacity, used both for a person who can drink an enormous amount without falling and, in the modern world, for huge volumes of data (海量信息, "a flood of information"). To call someone 海量 at a banquet is half compliment, half dare.

See the three water dots of 氵 on the left, and let them pour rightward into 每 until the whole character brims with water. That is 海: the place where every river ends, the widest thing in view. Hold the image of water gathering and you hold the meaning.

Now let the sea stand for whatever is too big to count. A crowd packed shoulder to shoulder becomes a 人海, a sea of people. A city that opened to the world by water is 上海, the town upon the sea. A person who can drink without limit has 海量, an ocean's capacity. Wherever 海 appears, ask what is being called boundless, and you will find the sea standing in for it.