Put your plane in place, then with it selected, go to the Material Room. Make the diffuse color black, and set the transparency node to 100%. Plug something in to the refraction node that varies the refraction over the surface of the plane (a black and white cloud texture should do the trick). From there, play with those settings to adjust their strengths... variations in refraction looking through water will be subtle, so a little bit of refraction change will go a long way, but it should make a noticeable difference in the "underwater" look.