I agree that the evolution of whales from land mammals to aquatic mammals is well represented by fossil evidence. For those that disagree with this, however, there is substantial evidence in other areas to further prove the evolution of whales from land to water. As can be seen on the website:
The Origin of Whales and the Power of Independent Evidence there is evidence in eight areas besides fossil records showing whale evolution. These areas include: morphological evidence, molecular biological evidence, vestigial evidence, embryological evidence, geochemical evidence, paleoenvironmental evidence, paleobiogeographic evidence, and chronological evidence. With these many separate areas of study all pointing towards a whale transition from land to water, it is almost impossible to argue that whales have always lived in the sea.