Make ths skybox a blue color. Then add a gui screen to the players make it either white or dark, and change the transparency of it to 0.6, or 0.4.
IT makes the screen darker and the transparency makes it seem underwaterish.
To change skybox, Lighting>Insert>Object>Sky. Just put the image link into where it says in properties. If you didn't get that at all, just try free models. Most likely a skybox isn't going to be infected.