Notice the scars on the back of the manatee in the picture. Some people chase or tease the animals. Some people discard fishing gear and other trash into the water. Manatees and dugongs accidentally eat this or become entangled in it and die. Floodgates and canal locks sometimes kill the animals by trapping them under water and causing them to drown.
Pollution kills the seagrass and other plants which manatees and dugongs eat.