Go through water without using up Airtank:
First, grab the airtank. While the airtank is still active, go to another room. Access the Sub-Screen Menu and choose Back, which puts you back into the room with the airtank before you took it. Die in that room. When you revive from your death there, you will be able to go through the water. In this manner, you can reuse airtanks, which are meant to be one-time use.
To access a secret sub-screen, while playing, just press either the A or B buttons on controller 2, this will bring you to a sub-screen menu. Once in the sub-screen, use controller 1 to move the heart-shaped cursor down or up with the directional button, and press the A button to confirm your choice. Options within this menu are:
Reset - resets everything within the current room, so you can try again (you lose one of your lives using this option)
Back - if you open the wrong door, you can start over again from the previous room (you lose one of your lives using this option)
Game - closes the menu without doing anything, returning you to your current game (the instruction manual for this game tells you that it resets the entire game, this is a mistake)