Yes it is extreme because you only have a certain amount of oxygen . If that runs out and you are very deep under water, there is a very good chance you can die or collapse.
If you don't have the right equipment on, like a broken depth gauge could make you very sick because you and sucking up bad chemicals from the equipment. You get this because it doesn't tell you the right depth and it could lead you to bad chemicals.
There is sea life that can mistake you for food, like sharks. When you have flippers on, it makes the shark think you are a giant fish. After that, it approaches you and tries to attack you.