The Dall's Porpoise, a small cetacean, loves to swim at high speed -- up to 30 knots -- just under the surface of the water. At such speeds, its head and back produce a bow wave called a "rooster tail," which creates a hollow cone allowing the animal to breathe while still under the surface of the water. It likes to "bow ride" with fast vessels, but if the vessel is not fast enough it quickly gets bored and moves on. Adults average 6 to 7 feet in length.