Because fish have gills and can breathe under water and humans do not
For oxygen masks for firefighters, scuba divers etc
By oxygen tanks - similar to SCUBA divers.
So that they may breath underwater.
Scuba regulators, also called 'diving regulators', are vital for scuba divers to properly obtain oxygen from a tank. Regulators are available for purchase through diving specialty stores such as Prime Scuba, House of Scuba, and Divers Direct.
The collective noun is a bubble of scuba divers.
They can't survive without oxygen but they can survive with an oxygen mask that scuba divers use and with supply of oxygen of course .
Naturally the air we breath has 70% of nitrogen and only 21% of oxygen, this is done is scuba diving as well. Just oxygen is not enough for living, while oxygen is essential one.
scuba divers
Many people use a SCUBA suit for diving. Sport divers, police divers and some Navy recovery divers.
Free divers do but scuba divers do not need to. Scuba divers take their air with them and would have no need to hyperventilate.
seebees
They swim