The fastest animal in water is the black marlin. Black marlins, a type of billfish, are known for their incredible speed, reaching up to 82 miles per hour (132 km/h). This impressive velocity makes them not only the fastest in their aquatic environment but also one of the top predators in the ocean. They possess a streamlined body and powerful tail, allowing them to swim effortlessly and hunt efficiently. Black marlins are often sought after by sport fishermen due to their size and speed, making them a popular target in deep-sea fishing activities.