Very very easy. I just painted a dark-greyish-blue colour on top then lowered the opacity until I could see your grid pop through. Then after that it is all shadowing. Compare your water in post 1 with mine above: they are pretty similar except the colour. The other, more subtle difference, is that I actually have two shadow layers on top of the water: a big heavy one coming out from the cavern walls, and a much lighter smaller one from the bridge. This is important because there are three elevations in this room (wall; bridge; water) and you need to show that with the shading.
How did you make the water? I hope I can mange to do the same; I really like it!