That way the dolphin does not need to slow down in the water. They glide across the surface of the water. They can glide very fast and very far through the air. As the fish leaves the water special fins are spread out that allows it to glide. They use their fins in a different way than fish.
Fish move their fins sideways.
The ray moves its fins in an S-shaped wave, up and down, from the head to the tail. Some water animals have flippers. Flippers can act like an airplane wing in the water. The sea turtle, sea lion and seal are examples of animals with flipppers. The movement of the flippers allows the animal to go up or down or forward in the water.