Swim on the water surface toward a shore. Before you end up swimming in water too shallow to dive, press Z and instantly start mashing B. You will slowly advance through the shore and end up swimming underneath the ground. After a while, stop pressing B. If you stop too early though, you might pop out of the shore and land on solid ground.
Use STS on the fairy island and go to the small platform behind the loading zone. Try to jump either onto the platform or straight into the loading zone. The latter is faster, but risks going back in bounds. The path on the platform is too narrow to properly walk on, so you'll need to jump repeatedly in order to get to the loading zone. Once you're inside, you can get the shockwave move and the camera.