So what do you do?
1. Are you calm cool nature guy, who says "did you see the distinctive pattern to its fin and how it spun in the air? That's a spinner shark...not dangerous to man."
2. Freak out and leave the water immediately.
3. Stay calm, but having noted that a nine foot long shark has just leapt out of the water choose this moment to take a break and watch your comrades play in the water. And see if any of them get et.
(First seen on Hudson River Windsurfer, who recommends Fishbase.org as a source of details on fish, sharks etc. If Peconic Jeff will start sailing with his iPhone, bookmark Fishbase and promise to drop his rig and look up sharks the moment they start leaping around us, I may put myself in category #1. Until that time I'll be spending time in answers #2 and #3.)