Manatees like many animals, have suffered from loss of habitat and overhunting.
Humans are one of the biggest threats to their existence.
Manatees like to bask in warm water just under the surface, and they are slow swimmers, this makes them very susceptible to being hit by speeding motor boats. They can be almost impossible to see and when hit by a power boat the propellers often cut them badly. Many manatees have died from boat related injuries.