Step-by-step explanation:
In addition to the vestigial leg bones and the series of transitional species in the fossil record, there is physiological and genetic evidence.
The physiological evidence is that whales swim with a modified running motion. If they had been specially created in the sea, there is no reason to have this holdover from life on land to let them swim.
Evidence for a horizontal fluke comes from the muscle attachments to move that fluke. Muscles pull on bone when they contract. Therefore bones have little bumps (sometimes not so little) on them as they add bone to handle the mechanical force of the contractions. Flukes can be inferred by the placement and size of the bumps on the vertebrae where the muscles that move the fluke (tail) are attached. Flukes themselves are soft tissue, and thus do not preserve well. But the bumps on the bone show a clear preference for muscle attachments on the top and bottom of the vertebrae, and these bumps get larger as the muscles evolve to be larger to move the fluke. If there is any other explanation and another inference, creationists have not provided it.
There is also the position of the nostrils over the course of evolution.