Even if you aren't hurt seriously when you hit the water, you're likely to become disoriented. The forceful impact combined with the depth you sink after jumping from a height could frighten you, making you want to draw in a breath quickly, which isn't safe when underwater. This could lead to panic, which in turn could lead to potential drowning. If you are injured during the jump, you're even more likely to drown, because it's difficult to swim with broken bones. You won't be able to swim at all if you're unconscious. Depending on your location, it could be difficult for rescue teams to get to you fast enough to save you.