As Mario, enter the Wet-Dry World painting at the lowest point to make the water the lowest inside the level. Once in, get to the top of the stage with the red "!" block but don't raise the water level. Get floaty and make your way to the tunnel that leads to the second area of the level. Touch the crystal that lowers the water and get the "!" block, then float back up through the tunnel where you came from. At the end of the tunnel, triple-jump to get up and you will suddenly hit water as if you were swimming. Swim down the tunnel and eventually the water will disappear and you will hit solid ground.