It either was not an accidental hit by the dolphin or it wasn't really harmed. They ram into sharks and have amazing echolocation and body control. I imagine that they are very aware of their surroundings and there are no uncoordinated dolphins. Going after a fish and risking ramming something is also quite believable.
I see them pretty much every time I go out. I found a place where they congregate to feed and go back there again and again. They have come up next to me quite a few times. My Adventure Island moves fairly slow and isn't a threat. Sometimes one will surface enough to see me (eye out of the water) but mostly they only surface to breathe. I've felt the spray from exhaling - pretty cool. I've seen babies as little as they get - about 2 feet long - like a miniature dolphin following along with the rest of the family / pod. I think the males circle around the females and younger ones like scouts in case of a shark or something. It is really cool being out there, my butt just an inch out of the water, no sound but the wind and waves, in the same elements with dolphins.
So no, it probably wasn't done on porpoise.