In Coastal Remains, by the piranha pond, glide to the place with the single indian guy. Close to the wall, there is a chest that requires you to horn dive into it to break it. Do NOT break it. It's the key to easily swim in the air
Now, jump on top of it, and close to the wall. When you get Spyro to believe he is falling, charge, and you will fall down between the chest and the wall. Try to walk into the wall, and you will go through. Below, there is water, but only close to the pond. Land in the water and dive.
Now you have a lot of possibilities.
Swim down to get into the tunnel that leads to Ineptune.
If you don't want to go there, try not to swim down.
The invisible walls are really annoying. The level even has an invisible roof
Swim in the air in Clody Domains:
Swim to the elevator that takes you to clody domains. Press triangle when the green triangle shows above the platform. You will go high up in the air while you go to Cloudy Domains. When you can see Spyro again, swim down, and you get to the level. If you want to keep swimming for a while, don't talk to Elder Titan.
The same is possible to do in Sunken Ruins, but as you have to go into the water down there, you won't be able to swim in the air in most of the level. When you have reached the bottom, you can swim up to where Coastal Remains should be, but the level isn't loaded, so only the entrance to Sunken Ruins is visible.
Have fun
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