DUGONG
The Dugong is the only marine mammal that feeds solely on vegetation (the Manatee could be another but they spend some time in the freshwater so they get disqualified).
Diver Tip:
If you want to catch a glimpse of tho massive mammal then I suggest spending some diving time in the northern waters of Australia, between Shark Bay and Moreton Bay where the Dugong likes to hang out.