There are multiple possible fastest animals in the sea due to the difficulty of measuring animal movement speeds in the open ocean; however, the fastest verified animal is the sailfish, whose leaps can propel it up to 68 miles per hour. Other contenders are the black marlin and swordfish.

Scientific calculations suggest that swordfish can swim up to 60 miles per hour, and some have claimed that they can reach 80 miles per hour, although neither speed has been directly observed. Claims have also been made that black marlins are capable of swimming up to 80 miles per hour, though this is also not based on direct observation. If unverified claims and leaping speeds are excluded, the fastest swimming fish in the sea is the shortfin mako, which has a top speed of 31 miles per hour. The fastest swimming mammal in the sea is the killer whale, with a top speed of 34.5 miles per hour.