Since different groups of marine mammalian originate from either different ascendent, this occurs as example of convergent evolution.

Since mammalian originally evolved ashore, their spines are optimized for running, allowing higher-&-down but only little crabwise motion. So, marine mammalian occasionally swim by moving their spine higher & down. By direct contrast, fish commonly swim by moving their spine crabwise.

Everthing mammalian stand hair but marine mammals, except polar bears & otters, develop wasted virtually all of their hair, so decreasing a body of water trend lines. Instead, for heat insulatiin it rely on the layer of fat.

A polar bear spends the big proportion of its period within the marine environment, albeit a frozen a single. Whenever it does swim within the open sethe; these are pleasantly skillful & has been shown to handle 74 klick in a day. For these reasons, a bit of man of science regard it as a marine mammalian.