I’m going to take an existing battlemap and turn it into an underwater ruin. Here’s the map I’ll be using – a simple ruin from this Ruined Library map pack. Ruins work well as they can easily be the remains of a unfortunate city subjected to an Atlantean cataclysm.

1. Add a colour layer
The first problem is that the map is definitely not the right colour. We need to desaturate the map, and add an over-all blue cast to the map. I create a new layer above the map and fill it with a grey blue (#3e526a to be precise). It’s not too saturated, so it’ll do both jobs at once. I set the layer blend mode to Color, and set the opacity to 75%. For more on blend modes, see this post. That gives this:

2. Add an overlay layer
Note that you can still see colour variation in the map – that’s because we set the colour layer to 75% rather than 100%. You don’t want to wash out all the prior colours. The map looks a bit like a moonscape rather than an underwater map. To bump up the blues a bit I duplicate the colour layer, set the blend mode to Overlay instead of Color:

Here’s the layer stack:

3. Adding Ripples
So we could leave it there, but I want to add some ripples to the map. As light hits the waves on the surface it is distorted and that creates a pattern of light and dark across the sea-bed that’s quite distinctive. Yesterday I covered how to create a ripple pattern. You can follow those steps, or just download one of my ripple files here (note the file is CC-BY-NC licensed, so if you want to use the ripples in a commercial product you’ll have to follow the tutorial and make your own!).
I select a large region of the ripples texture and paste it into my working map – it will show up as a new layer. Once there, use the transform tools (ctrl/command – T or Edit->Free Transform) to spin it round and make sure it covers the whole image. Add a color layer above it, find a nice blue and merge down so that the black parts of the texture are now a good sea-blue color. Finally I set the blend mode to Multiply and drop the opacity right down to 30%:

4. More Ripples!
You can immediately see the difference that the ripple pattern makes to the map. I like this so much, I’ll do it again. The second layer is coloured with a greener blue to add some colour variation to the map, and it’s been rotated so that the ripples aren’t going in quite the same direction:

And we’re done! The ripples give a convincing under-the-sea feel to a map that started off as an arid blasted desert ruin.

You can find more tutorials under the Tutorials section of the blog
This tutorial is also available in video!