xkcd

If that weren't the case, you really only have two options:

• Sharks maintain their height out of the water via using their pelvic fins or similar to generate lift via Bernoulli's principle and are required to thus maintain very high speed as long as they want to stay out of the water. Think of a hydrofoil, but a shark, sharks cruising around skimming the water at a hundred miles an hour or something.

• The shark has a dramatically-lower density than water, has a greater-than-neutral buoyancy, which I suspect would create difficulties with crushing when the shark wants to dive deep.

Knowing Randall's "What If" history of giving real scientific explanations that extend into the lunatic consequences of a scenario, I'm surprised that he didn't jump on that in the comic.