>>1 If this is a final destination sort of thing, I've got 1 idea.
One character does something stupid that results in them falling off a boat and into the water. Another character jumps in and swims after them. It'll make the reader believe that the first character is going to die, but they actually get saved by the second one and are returned securely to the ladder of the boat. the second character also scales the ladder, and the reader will think "oh, false alarm", right? The engine falters on the boat, and the jolt knocks the 2nd character back into the water. He swims back to the ladder and manages to reach one arm out... But then at inhuman speed he's jerked far to the left. Then far to the right. His face and upper body are scraping against the metal sides all the while. The event looks as disturbing and supernatural as movie portrayals of people who are possessed.
The other characters call out to him, screaming what's wrong. When he lifts his face it's covered in blood, and a pool of red water surrounds his body. That's the last they see before he's pulled under water, large dispersal of red clouds float to the top of the water where the second character once was. The red then trails, a slightly wavering line that seeps away from the boat. And before it's fully out of sight, you see a grey fin surface the water before dipping back down.
But if you wanted something more supernatural, then just turn the shark into a ghost, or something?