Fog at a certain depth

OK, so i am making a game were you are a fish. When your above water it is clear, when you below water it is still clear. How would i make it so when you go under water it is a bit foggy. Is there a way (maybe using the mist function) to make it so the mist is only at a certain depth?
Or any other ideas.

You could use a fog filter, enable it when the camera is below the water level, disable it when its above. Martinsh wrote one here: I’m not sure if the fog properties are available in the API. I havn’t come across them yet but they may be hidden in there somewhere.