As well as the Boost, useful for catching food, your Sharks are able to jump out of the water to catch seabirds and, if your timing and execution is impeccable, parasailers.
The best way to execute it is to line up your shark beneath the target you are aiming for and begin your ascent. As you near the surface, use your Boost and your shark should burst out of the surface of the water in slow motion, grabbing and eating any birds or people on the surface of the water and above it.
Parasailers are much higher up than anything else in the game so you will have to be patient and wait for the right shark before you will be able to catch them. If you level up the Boost and speed abilities of your shark that should help - otherwise, wait for the parasailer to lower himself for an easier meal.