Have you not heard of the ship's mate? This is from Chambers:
mate1
n a companion; an equal; a fellow worker, occupant, etc (often in combination, as workmate, flatmate, etc); a friendly or ironic form of address; a husband or wife; an animal with which another is paired to produce young; one of a pair; a ship's officer under the captain or master; in the navy, formerly a rank to which a warrant officer might be promoted;