Go to Aquaria Towers and after you push all the hatch buttons, go to Hunter's stingray area. Swim to the barrier that prevents you from going out of bounds and charge in a circle around the area, be sure that Spyro is pushing against the barrier. You'll see him slowly work his way through and will eventually pop out on the other side. Here you can see and swim through walls and access any point in the level by swimming to it.