The inert gas which is substituted for nitrogen in the air used by deep sea divers for breathing is

In deep-sea diving, helium is the inert gas substituted for nitrogen in breathing mixtures to prevent nitrogen narcosis and decompression sickness [t2][t4]. Nitrogen, which makes up approximately 78% of normal air, becomes narcotic under high pressure, impairing a diver's judgment and motor skills—a phenomenon known as 'rapture of the deep' [t1][t6]. Helium is used because it has significantly lower narcotic potential and lower solubility in body tissues compared to nitrogen [t2][t6]. Common breathing mixtures include Heliox (helium and oxygen) and Trimix (helium, nitrogen, and oxygen) [t3][t5]. While other noble gases like argon, neon, and krypton are also inert, they are not suitable substitutes; argon, for instance, is more narcotic than nitrogen under pressure [t1][t7]. Helium's low density also reduces the work of breathing at depth, making it the standard choice for technical and deep-sea diving operations.

Sources

1. [1] https://www.ncbi.nlm.nih.gov/books/NBK470304/