Question

Why deep swimmers use special suits while swimming?

Answer 1

The pressure exerted by water in the oceans increases with depth.

Very deep down, the pressure is enough to crush the human body.

That is why deep sea divers wear special suits which can withstand such high pressure and prevent their bodies to crush.

Answer 2

At significant depth, divers might use an Atmospheric diving suit like this: as opposed to a wetsuit, or a Dry suit like this: The atmospheric diving suit is rigid, strong enough to maintain normal air pressure inside against dozens of atmospheres of water pressure on the outside.