Ok Bruce, here is the reminder Basic Marine Biology 101 class (advantages of having a H who majored in Marine Biology in college, even if it was donkey's years ago):

Sharks must swim... or they sink. They are not as bouyant as fish, which have a gas-filled bladder to aid them... All a poor shark has is its oily liver as an aid...

Shark swimming is more like flying than swimming.

Some sharks are fast-swimming predators that live in the open ocean, some are slow-swimming bottom dwellers that eat shell fish, and some are slow-swimming filter-feeders that sieve tiny animals and plants to eat.

Unlike fish, sharks cannot stop suddenly or swim backwards. A shark's pectoral fins cannot bend upwards like a fish, limiting its swimming ability to forward motion. If a shark needs to move backwards, it uses gravity to fall, not swim backwards. Sharks must swerve to the side in order not to hit something - they cannot simply stop.

Some sharks (like the great white shark -that is you, Bruce) swim by propelling itself through the water using its tail. The fins are only used for balance. Other sharks, like the whale shark, move their bodies from side to side to propel themselves through the water.

And always remember, regardless of speed, a shark must keep swimming or it will sink.

Clear now on why you must keep swimming? Any lingering questions?

I thought so

"You don't throw a whole life away just 'cause it's banged up a little" Tom Smith in "Seabiscuit"