At sea is used for being on a vessel away from land. At the sea (British) is the equivalent of at the ocean (American), which mean in the vicinity of a sea, such at a port, or a beach. "We are vacationing at the sea"
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Life at sea
Life on/at the sea
Imo.
The 2 Have subtle but different connotations and different affect centers. First is locational, the Second implies movement and occupation.