The evolution of cetaceans is one of the most well documented in all of evolutionary science. But most of the info I find exclusively discusses the morphological changes that allowed them to adapt to aquatic life.

What I want to know is about the selection pressure. The earliest cetaceans, from Pakicetus to Ambulocetus, were primary suited to freshwater and brackish water habitats, while later cetaceans in the family Basilosauridae inhabited salt water. What evolutionary niches were open for them to exploit, and what selection pressures pushed them to be predominantly oceanic animals rather than fresh water?

Clearly there were distinct advantages that meant they found success in aquatic environments, or else their hind limbs wouldn't have become vestigial. Given how they have come to inhabit just about every marine environment today, I'd love to know what environmental factors lead to their success.