Attached Link: http://www.renderosity.com/gallery.ez?ByArtist=Yes&Artist=DryFly

Your best friend when building a UW scene is haze, fog and volumetric slabs. If you want, or care to :), you can take a peek at my gallery and you'll see that in all of the UW scenes I build, even in the clearest of alpine lakes, there is still a fall off to almost zero visibility at some point. This takes place pretty quick beyond a certain distance. Also keep in mind that light tends to blend with the turbidity and color of the water which results in a washout. (yours has a great start on this) This effect is increased exponentially by depth. Especially in the ocean. Beyond a certain depth it is nearly impossible for the human eye to discern anything less than extreme differences in color, and even then usually only in darkness of shading. So take into account just how deep you want your picture to be and increase the atmosphere to reflect (and decrease the intensity of the caustics). You have a great eye for veiwing angle and you picked a tough one, because your visibility will have to drop off vertically. :) Although the intensity of your caustics would lead the eye to believe it is relatively shallow, so it's a trade off. Lastly, if you choose to use volumetric primatives for light as Stefano (Doublecrash) advised, then the intesity should also be lessened the deeper the image. I never claim to be an expert, by any means, but I have done ALLOT of UW (albeit mostly freshwater) so your welcome to take, leave or stomp all over my rambling advice. :) A great start and a very challenging angle, I look forward to seeing the finished piece. Matt