I'm trying to make a pattern for underwater mountains using these mountains and water templates, but I don't know enough about gimp to do it well. Any advice?

Here's the two original templates, and my best result
+ =

I did this by deleting the gray out of the mountains image, but it doesn't look great.

Also, here's a map I've finished using these templates. I also want to do the same thing with the plains and forest templates used in this map. (Apologies to anyone living in Denmark for stealing all your city names).