What defines a marine mammal?

Question

What defines a marine mammal?

Answer 1

A "marine" Mammal, as opposed to an "aquatic" Mammal, is one that lives in the ocean.  It is an animal that is warm-blooded, has a vertebrae, has hairs (even extremely small ones), and nurses its young.  But, it also lives all, or most, of its life in sea water.  

Examples of this are:  Whales, Dolphins, Seals, Sea Lions, Walrus, Elephant Seals, Otters,etc.

Aquatic Mammals live in fresh water lakes, ponds, and rivers.  These are Fresh Water Dolphins, River Otters, etc.

Answer 2

A mammal that lives in the water, and have lungs...need to breathe air.

Answer 3

A Marine mammal is  easily identified by its short haircut and shiny boots.  They have a strong protective attitude and a proud heritage.

Answer 4

a mammal is determined mostly by the fact that we give birth to live young, are warm blooded and of course breathe oxygen in and release carbon dioxide (oh my god we breathe out carbon, what a shocker don'tt tell the greenies they'll tell us to stop breathing). marine mammals such as whales, dolphin, and sea lions need oxygen to breathe much like our good selves,and give birth to live young, there are only 2 families in the mammal genus that do in fact lay eggs - the echidna and the platypus. also the fact that yes indeed mammals do tend to be covered in hair (except Whales and Dolphins which lost theirs through evolution)