[Gimp] How do I merge these patterns well?

[Payback]

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).

[Eathanu]

Hmm... Maybe you could try Colour to Alpha from the Colours menu, then set it to the grey background of the mountains (41413f) and copy the result onto the water? That should give you the black lines just fine, while getting rid of the grey background. Otherwise, you could do it the other way around, but if it's underwater mountains, then I think maintaining the blue is the better option.

[Payback]

That looks awesome, thanks.