Diver

n.
  1. One who, or that which, dives.
  2. Fig.: One who goes deeply into a subject, study, or business.
  3. (Zoöl.) Any bird of certain genera, as Urinator (formerly Colymbus), or the allied genus Colymbus, or Podiceps, remarkable for their agility in diving.

The northern diver (Urinator imber) is the loon; the black diver or velvet scoter (Oidemia fusca) is a sea duck. See Loon, and Scoter.