The current thinking of why mammals live in the sea is based on the hypothesis that mammals evolved on land and marine mammals are a lineage of mammals that returned to the sea. Please watch the PBS video found at the following link:
https://www.pbs.org/wgbh/evolution/library/11/2/e_s_3.html
or
https://www.youtube.com/watch?v=ewtw_nZUIDQ
Evolution is still a controversial subject in the States (not so much in Europe). What do you think? Provide at least 2 references to support your opinion. (Please note that The Bible is not a scientific reference, there is research there if that is the route you take.)