Is your question something like, Would evolution allow animals to live in ways that really don't seem efficient to us?  Why should whales live in an environment where they could easily drown?

Fish and whales (and all 'common' living things) need oxygen to survive but they get the oxygen in different ways.  Whales do very well (or at least did in the past) living in the water and having to come up for air.

Survival of the fittest means: in a group of living things those that are best adapted to the current situation will (on average) have more offspring and pass on more of those genes that allow them to survive and reproduce.