I've been listening to the trial at work and there's alot more context that's not shown in the video.
It sounds like the dude was looking for a phone, the teens seen him and started making fun of him. One of the teens pulls out the camera and starts filming, that's when the video starts. There was also testimony that one of the teens deleted another video of the incident.
According to the defense, the old dude heard the teens screaming at him and saw one holding up a phone so he ran over thinking they found his phone. He lost his balance and fell into their tubes then dropped his snorkel, you can see this happening in the video.
The teens start yelling about him looking for little girls, that caused a group of adults to get involved. One of the adults was the girl yelling in the man's face and later got stabbed, Another adult was actually the first to attack, you see him slap the man, he also got stabbed.