Helium – oxygen mixture is used be deep sea divers in preference to nitrogen – oxygen mixture because

Reduced risk of nitrogen narcosis: Nitrogen narcosis is a condition that can occur when nitrogen is breathed at high pressure. It can cause symptoms similar to those of drunkenness, such as impaired judgment, confusion, and euphoria. Helium is an inert gas and does not cause nitrogen narcosis, which makes it a better choice for deep-sea diving at depths greater than 50 meters.

Reduced risk of oxygen toxicity: At high pressures, breathing pure oxygen can cause oxygen toxicity, which can result in seizures, nausea, and other symptoms. The use of a helium-oxygen mixture reduces the risk of oxygen toxicity because the partial pressure of oxygen in the mixture is lower than it would be in a nitrogen-oxygen mixture.