Wikipedia confirms that only Steller’s sea cow got that big
[QUOTE=Wikipedia]
Living sirenians grow between 2.5 and 4.0 meters long and can weigh up to 1,500 kg. Hydrodamalis gigas, Steller’s sea cow, could reach lengths of 8 m
[/QUOTE]
Reading the article reminded me of something else- Sea cows are coastal animals. If you’re out in the middle of the ocean, far from human habitation, you’re also far away from sea cows. If you’re in a region with sea cows, you could probably find humans.