This is one of those old cartoon tropes when a character falls in the water, and then he sees a fin sticking out of the water (may not be just one) and propmtly imagines "Oh Crap!, a shark! Gotta beat it!". However, most of the time it's just someone pulling a prank by swimming under the water with a fake shark fin strapped to their back. Or it may be a harmless dolphin. May be subverted when the character's Genre Savvy enough not to fall for it, but then it turns out to be a REAL shark.
Hello, Unknown Troper. You'll need to get known to lend a hand here.