A number of fish can, but usually don't. Those that can are mostly members of one of the eel families.
Sharks lack the ability to swim backwards and the ability to stop swimming suddenly. If a shark needs to move backwards, it uses gravity to fall. In order for a shark to come to an abrupt stop they must swerve to the side in order to hit something as they cannot just simply stop themselves.
If sharks do not keep swimming they will sink. The reasoning for this being that unlike other fish they are not equipped with a gas filled bladder to help keep them afloat.
For whales and dolphins, they have flippers used for steering, for balancing, and for stopping. But they are not used for moving forward through the water. Forward motion is created by the whale’s or dolphin’s tail, or flukes, moving up and down in the water.
It serves as the propeller. Powerful muscles running along the backbone and sides of the whale’s body move the tail up and down in the water, providing the power that pushes the animal through the water, or deep into the ocean.
Think you know everything about fish? Check out these 17 crazy facts about fish shared by Swallow Aquatics.