Because if you try to dive to the bottom of the ocean without an oxygen cylinder, you would have to surface again before you even get all the way down. Furthermore if you did manage to get to the bottom you would have to swim back to the surface for air and if you cant get to the surface in time you could very well drown by water filling up your lungs.
if you're talking about the big cylinders they carry on their backs, then that is their main supply of oxygen. if you're talking about smaller ones that are usually strapped around their waists or attached to their "backpacks", then those are in case a fellow diver runs out of oxygen and they need a small amount of emergency oxygen or you yourself run out and need something to help you until you return to the surface again.
For a way to breathe underwater.
You can buy a diving regulator online at Divers Supply, Leisure Pro, Dive Right, and House of Scuba. You can also purchase a diving regulator on eBay and Amazon.
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 .
With scuba diving, you are diving with an oxygen tank on your back and a regulator in your mouth. For SNUBA, there isn't an oxygen pack on your back, instead your oxygen supply is a long hose attached to a raft on the surface of the water.
You can buy diving gloves at most underwater sports stores. There are many on the web such as Leisure pro or Divers Supply. It's worth also checking out the usual suspects like Amazon and eBay.
They make their own oxygen by mixing nitrogen with carbon dioxide.
They have oxygen. And they float. Unlike your brain.
they were special kinds of suits which are in-built with oxygen supply. because there is no vacuum in space, they can float, making them go to any place they want.
a breathing tank! try it its fun! (underwater swimming)
Perhaps you are referring to a breast plate. A bp is what commercial divers wear that allows them to go under and work. Sport divers don't wear complicated gear. Commercial divers wear an outfit that protects them from a harsh environment. Hardhat, breast plate, suit, shoes, wt. belt, air line to the surface with unlimited air supply according to the depth they are diving at.
Astronauts carry air tanks with them that contain mostly pressurized oxygen and nitrogen. Their suits circulate the air to their helmets and throughout their entire suit so that they can breathe.They breath threw oxygen tanks tied to their backs. If an astronaut was to take it off and try to breath, he/she would start to choke and die.
The Lunar Module and the astronauts' spacesuits carried plenty of oxygen for the Moon landing, but they were critically low on fuel to leave.
Space ships are pressurised and contain their own atmosphere of either oxygen or air (NASA uses oxygen, Russia prefers air). When astronauts have to go outside the ship, they wear spacesuits which are essentially personal spacesuits with their own oxygen supply.