The human body isn’t exactly suited for freezing water. So as soon as you enter the ocean (or frozen lake, river, pool, etc.), you go into something called “cold-shock”, which is an involuntary response—meaning you have no control over it—that causes you to take a giant gasping breath in, says Joseph Herrera, the director of sports medicine at Mount Sinai Medical School. This can be incredibly dangerous; since you have no control over how and when it happens, it could occur when you are underwater, causing you to take water into your lungs. Depending on how much water you inhale and how long it takes you to get back to the surface, you could end up drowning.