Super Mario World/Vanilla Dome 2

To get to the red switch house: Go until you get to the single question box which gives you a fire flower. When you hop on it to get out of the water, jump to the right and then left. Go a bit, pick up the P-Switch, and go as far left as you can. Activate it, jump over the first hole and into the hole with the coins. Fall to the topmost ledge for the key, then go to the ledge underneath for the keyhole. Watch out for the three fish around the keyhole!