This probably works with any character, but has only been done with Mario. Go to Jolly Roger Bay, and head to the middle of the water area (by the boat) where there are long poles coming out of the water. They have thick bases that become narrower at the top of the pole, making the top part climbable and letting you reach coins or extra lives. Go to the one by the wall (this may work with others). Climb up it from the base slowly and you will make a little bit of progress. Because Mario's not meant to climb up things of that thickness, his hands will go through the pole and after a little ways up, he'll eventually release his grip and slide down. After sliding down, often Mario will end up standing on top of the water. Walking around will make him fall back in, though.
Submitted by: kornyhatemachine