I don't know how interesting or useful this would be to anyone but when working on the april comp I found a quick and simple way of creating an underwater effect for maps, thought I might share. I have provided five pics to show what you should have as the effects are applied, can provide screenshots of settings and such if people need them. I did this in Photoshop CS3 so it should work in anything more recent.
Step 1 (Pic 1) - Create a map like you would at any other time. Then place ALL layers of the that map into a single group. Duplicate this group and rename the group "Water Base", now right click on the group and select "Merge Group". You can hide the original group to free up processing speed as you won't need them after this point.
Step 2 (Pic 2) - With the Water Base layer selected go down to the bottom and click "Create New Adjustment Layer" and select colour balance. Move the top slider all the way to the left towards Cyan and move the bottom layer all the way to the right towards Blue or set the Colour Levels to -100, 0, 100. Now go back to the water base layer and add another adjustment layer, this time Brightness/Contrast. Set it to -75 brightness and 75 contrast. Merge the three layers.
Step 3 (Pic 3) - Create a new layer and name it "Water overlay". With black and white as the default colours go Filters > Render > Clouds. With that layer still selected go to Filter > Artistic > Plastic Wrap. Set the settings to Highlight strength 15, Detail 5, Smoothness 15. The clouds should now look sort of like Pic 3.
Step 4 (Pic 4) - Simply set the Water Overlay layer to overlay
Step 5 (Pic 5) - This step is completely optional and is just something I do to add some more dynamic lighting. Select the Water Overlay layer and go Filter > Render > Lighting Effects. Using the default settings adjust the circle so that light is pouring in from the direction of your choice (top right for me) and make it so there is only a suggestion of shadow on the two adjacent corners.
Voila, done Hope someone finds this interesting.
NOTE: Using this effect most reds and browns will be drowned out so either add them post-effect or make them bright or else they'll turn black or grey.