[Gimp] How do I merge these patterns well?

[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.