Its not really a cheat, but after Tidus is rescued from the ruins by Rikku and her Al Bhed family and friends, you will be prompted to dive off of the ship and enter the airship. The guard closest to the save sphere will give you x3 potion when you talk to him. If you dive and return to the ship after leaving the screen he will give you another 3 potions. Its an easy way to build up potions as well as recieve spheres from the fish in the water. You can also take this opportunity to steal grenades for later use in the game.