What are the different grades of scuba air?

Answer 1

There are a number of different gases for diving.

Most commonly, air is used for recreational diving. Sometimes, nitrox is used which is a higher proportion of oxygen. This is usually 30-35% oxygen instead of the ~20% in air. This has the benefit of less nitrogen so less "on-gassing". This increases the bottom time and reduces the decompression stops.

For commercial diving, there are a number of other gases used such as Trimix which I believe has Helium in it.