Nicklen's caption tells the story shown by the footage:
US Fish and Wildlife and most tour operators have a strict code of ethic to respect the Manatees and maintain a safe distance while the manatees attempt to stay warm. The perimeter floats are put in place to keep people out but the Manatees go to where the warm water is and not necessarily within the confines of the rope barrier. They are literally trying to stay alive and it is imperative not to disturb them. However, it is hard to keep an eye on everyone and many disregard the rules. You can see people swimming amongst them, grabbing them and even pushing off of them. Then, more boats arrive and eventually there is a large bang from one of the boats and all of the manatees disperse back out into the cold water.
Nicklen noted that although these interactions are unfortunate for the manatees, he is inspired to shed light upon the negative effects of human interference and hopes to "drive the conservation debate" forward.