In air there is large amount of nitrogen and under high pressure solubility of gases increases. So, when sea divers dive in the sea then nitrogen dissolves in their blood and when they come up its solubility decreases and it separates in the form of air bubble which is very risky for the person.
So, helium gas is used in diving apparatus as it is less soluble under high pressure. It will not dissolve in blood in large amount.
Final answer:
Helium is used in diving apparatus because of its less solubility in the blood.