Pretty nice Olof, but I'd like to suggest use less Moon Rays, use multiple small regions seperated and placed almost randomly, and set them to moonlight, it will give a nicer looking effect imo.
The only problem with this is if you have units that are not stationed originally in the water, such as a Naga where you cannot have its swim animation play...which will make your underwater scene lose the effect..basically
The naga swimming through the water can be done with a couple triggers that aren't too hard, have been experementing with it for awhile in the World Editor for some of my series. I'd look at the triggers and get them now, but I don't have Wc3 installed atm since I recently formatted the computer.
I'll get a screen of something like it a bit later..if I remember.