The Answer:

According to Guinness World Records, the cosmopolitan sailfish is considered the fastest fish in the sea.

They point out that measuring a fish's speed over a short distance is very difficult to do. Still, in a series of speed trials carried out at the Long Key Fishing Camp in Florida, one sailfish took out 300 feet of line in three seconds, which is equivalent to a velocity of 68 mph.

—The Fact Monster