From the game Super Mario Sunshine for the Nintendo Gamecube
Go to Sirena Beach with any episode. Find a barrel and bring it to the ocean. Put the barrel down at the point where Mario stops walking and starts swimming. Stand behind the barrel and pick it up. Mario's height will go down as he kneels and you will be able to walk underwater. It may take a few tries to do. If you throw the barrel underwater, it'll make its usual above-water splash... so it'll be like water underwater. If you do a high jump, you can return to the surface and swim again.
Submitted by: DeathBorn 666