This is a glitch in Super Mario 64. To do it, go to Shifting Sand Land. Get in the cannon (you can get yourself to 1/2 health first, not necessary). Aim al the way up and left. Now, when you fire, you will hit the barrier, fall down, hit the edge of the cannon and lose half your health. If you had all your health, (i.e. you are still alive), repeat. You will have done it right when you are out of health but in the cannon. Now, aim in any direction and FIRE! No matter where you land, you will fall over and die (even on quicksand!) But, if you manage to land in the water, you will drown, yet still be above water. For added awexomeness, use the wing cap prior to dying. When you shoot yourself out, you can fly around, even with no health! Collecting coins won't restore your health. You're doomed.
Why it Works Edit
Even though you have no health, Mario can only die on solid land or in water. Since you are in a cannon, you are still known as "alive". As you hit the ground, the game switches you to "dead". The game doesn't let you collect coins or stars, as it has a checksum preventing that (say, like, you died, then fell on a coin.)