This mod allows the camera to go below the surface of the water, and has some effects including blue fog, caustics (those strange light things projected down).
There are no settings, simply enable the mod and get going. The majority of the mod was made by me, WNP78, and the caustics images were provided by Gestour.
Crafts used in screenshots:
MSC Vengeance by EternalDarkness
rocket biplane made by me for testing. (This shows how to use rockets for propulsion underwater, as they work for some reason. You need to attach a rocket to the back end of the detacher and the craft to the wide end, leaving nothing on the thin end. This will cause it to stay attached to the craft.)