Alright, here's what I whipped up for a little while after I woke up.
-I took your mountain background and edited it. With a flat lake-like area under the mountains, there should not be so much 'background mountain' and more 'foreground mountain.
1. I made the front mountains taller.
2. I added hill zones.
3. I added a treeline at the horizon point.
4. I added a reflected mountain area under the mountain.
The result is this:

-I took a look at your water texture. I did not edit it. BTW, it's HUGE! But that shouldn't be a problem for you, hehe.
-What I did do, however, is create a new gradient. You can probably just have GameMaker make a translucent alpha-blend, but here's the gist of it:

You place this gradient over the lake texture, as well as over the empty area at the bottom of the screen and in front of the mountain area that is reflected (I think I made it 16 pixels up - it's well-marked on the mountain graphic as the treeline is quite darker).
Now here's where things get interesting, because depending on how you set up your transparencies with the lake and the gradient, you can have multiple effects.
If you set the water texture to 50% transparency (which you should do 'cuz it looks pretty great),
AND
if you set the gradient to overlay mode (I don't know if GameMaker can do blending modes, but I'm assuming it can) and 50% transparency, you get this:

Pretty neat, huh?
However, if you do the same thing with the water texture
BUT
you set the blending mode of the gradient to "Normal" instead of "Overlay" and again set it to 50%...

Shazam! You now have Misty Water.