xkcd

The air we breathe is 21% oxygen 78% nitrogen and 1% other stuff. For scuba diving increased pressure at depth on boards more nitrogen in your body than normal so when you surface that extra nitrogen can form bubbles in places which shouldn't have bubbles and that can lead to injury/ death.

Scuba diving is a lot of fun. I encourage a9nyone vaguely interested to ho to their local scuba shop and do a demo dive.

Helium is lighter than air. Nitrogen dissolved in water leads to eutrophication, where the watercourse encourages bacterial and algal growth. Also too much nitrogen will kill you, so the diver will be the nutrient source, is I think what's implied.

There is an amazing audio tape where an astronaut who has been living in a helium enriched underwater laboratory for a month gets a phone call from President Johnson.