The lights were out, and you had your flashlight on you. Your flashlight was running out of battery when you saw blue lights in the water. You shined your flickering flashlight onto the water where the blue lights were; it was a blue shark fin.

Without another second, a head pops out of the water; it's a blue shark with a faint blue glow behind it. Then it popped its head back in the water and swam alarmingly fast towards you.