Kai

The name Kai has a Greek origin, where scholars link it to Gaius, an old Roman and Greek personal name whose sense had nothing to do with water. That is the root a dictionary will point you to first.

The meaning most people attach today, sea and ocean, arrived by other roads. In Japanese, Kai can be written with the character for sea, and Scandinavian families carried their own short Kai for centuries. In Hans Christian Andersen's story of the Snow Queen the boy is called Kay, often written Kai in translation, which kept the name close to home in the north. German usage picked it up too. So the water sense was fixed onto the name later, through the places that took to it, rather than handed down from the Greek.

On a Polish class register, Kai sits short and level next to longer names. It has no diminutive of its own and no separate girl's or boy's variant, so a child keeps the whole of it from the first day of school. Three letters and one syllable, with the plain meaning underneath: sea, ocean.