This paper investigates the term “sea” (hai) in the four-character Chinese idioms according to conceptual metaphor and metonymy theory, attempting to illustrate their conceptualization, determine their possible underlying motivations, and explore Chinese maritime thought and culture. Based on idiomatic expressions, three types of conceptual metaphors are identified: abstract qualities of concrete entities are the sea, abstract entity is sea, and a certain aspect of a human being is sea. Moreover, the four types of conceptual metonymies are the part for the whole, the whole for the part, the place for the product, and the place for the responsible deities or goddesses. They are motivated by a culture of worship of and accordance with nature, the pursuit of achievements in traditional Chinese literature, “man paid, nature made” as the attitude towards the ups and downs of life, and a self-centered conceptualization of the world. The maritime culture represented in these conceptualizations comprises fear of and respect for the sea, harmony between humans and the sea, and static–dynamic integrations of river, land, and sea. The findings show that the motivations of these conceptualizations do not only originate from the embodiment and Chinese philosophy of the unity of heaven and humanity but are also constrained by the most influential talent selection mechanism, the Imperial Examination System, as well as by agriculture, the foundation of the economy in ancient China.