Archaic term for a deity of the sea, sometimes equated with Poseidon or Neptune. Cross-referenced with 海神 (かいしん) sense 1.
See also: 海神 (かいしん)
In old myths, 海人 was used as a word for a sea god.
海神 (かいしん) is the more standard modern way to write 'sea god,' while 海人 (かいじん) is an archaic variant used for the same deity.
counter for people
The exact derivation is uncertain; the kanji 海 (sea) and 人 (person) combine to suggest a person of the sea, while the sea god sense may be an extension or separate usage.