Use A Capsized Boat As A Submarine
Explanation: Capt. Jack Sparrow and his crew in Pirates of the Caribbean: The Curse of the Black Pearl snuck up on their enemies in one scene by submerging an upturned rowboat and breathing in an air pocket in the upside-down boat's cavity. But MythBusters Jamie Hyneman and Adam Savage tried to recreate this scene and quickly sunk the pirates' cinematic submarine strategy.
First flipping over a rowboat on top of the water, Jamie and Adam easily floated along and breathed in the airspace between the sea surface and the boat. However, the movie pirates swam with the boats completely underwater. To do so in real life, Jamie and Adam demonstrated that you'd have to overcome the rowboat's upward buoyancy - the force that keeps it afloat - in order to drag it down into the depths.
But according to the MythBusters' calculations, it would take roughly 2,000 pounds of force to hold down the buoyant boat, making this feat pretty much impossible for mere mortals to pull off.
So unless we can assume that Capt. Sparrow and his gang have superhuman strength, the pirate submarine myth is blatantly busted.