When did whale ancestors begin living full time in the water?

It was 50 million years ago that the ancestor of the whales began to lived on the edges of the ancient Tethys Sea according to the exhibit in the museum of New Zealand.  The ancestors of whales were living in the land, but were mainly eating fish.

Basilosaurids are ancestors of the whales that inhabited the oceans approximately 38 million years ago. They did not live on land and were 18 meters long. they were discovered in 1840 and initially confused with a reptile.

They existed from the end of the Middle Eocene to the beginning of the Upper Eocene. The researchers believe they were the first fully aquatic cetaceans and gave rise to modern cetaceans.