I think it's interesting that aquatic mammals come from many different places on the mammal tree. (Down to the platypus, which is basically as far evolutionarily from other mammals as it's possible to get.) Returning to the water seems to be a common strategy. Manatees are related to elephants. Seals and otters are close to each other but much more closely related to land predators than to any other aquatic mammals.
Manatees..? Have to look that one up :-) Thanks for this enlightening addition @tcpolymath :-) Platypus!! Duck-bill mammal that lays eggs... I should do a post on that one sometime. Australia is an evolutionary playground of the nicest kind :-)