A plant-eating mammal that lives under the sea is called a sea cow or a manatee.

There are also other marine mammals that can be herbivores, but they are not typically called "sea cows":

* Dugong: These are closely related to manatees, and are also herbivores.

* Some species of whales: Some baleen whales are filter feeders and eat mostly plankton, which are tiny plants and animals.