Like fish whales live in the water. They have fins. Sure, you can. Whales have horizontal tail fins. They are from side to side like wings of an airplane.
Fish have vertical tail fins.
They are up and down. Like whales, other mammals also live in the water. Some can not go on land, some can. Walrus, Sea lion, Seal, Manatee, Dugong are all mammals who live in the ocean.