They eat vegetation. But they are so big that my first encounter with one really scared the un-rational side of my brain. I'd be on top of it. I'd never forgive myself. These fears disappeared in 10 minutes.
Manatees are indeed big, but that's one of their defense mechanisms.
In no time, I was practically one of them. A small manatee approached a member of the group. The manatee put its face up to the man's mask. Then the calf swam over to another person not from our group. The manatee seemed to be introducing itself to each person there.