Flipper

forelimb evolved for movement through water

A flipper is a special kind of limb that is flattened like a paddle for moving through water. Many animals that live in water, such as whales and seals have flippers. The flipper of a fish is called a fin.

A flipper of a Humpback Whale
Swimmer's flippers

It can also be an item worn on the foot to help people swim more easily.