is a specific kind of: crewman, sailor.

is a broader term that covers: able-bodied seaman, able seaman | bo's'n, bo'sun, boatswain, bos'n, bosun | deckhand, roustabout | helmsman, steerer, steersman | bargee, bargeman, lighterman | officer, ship's officer | pilot | sea lawyer | whaler.

derivation: seamanship | seamanlike, seamanly.