What is the only entirely marine mammal to feed exclusively on plants?

There are a few sea turtles that are mainly herbivorous. Such example is the green turtle. This turtle is a devoted herbivore, grazing mainly on sea-grasses as well as algae. Much like the green, we have manatees and dugongs. They are rare, only three kinds of manatees exist and there is a single type of dugong. They are even more unique because they are, just like the turtle, completely herbivorous. There are more examples, but I hope this is enough!