Thanks for the feedback!
I removed the shader and improved the default material of the lava. Maybe I will "add a heat distortion shader" with an option in the main menu to turn it off.

Yes, I know, the graphics (still) suck. I'm still coding and developing various stuff, I did not work on the graphics yet (except for the lava/water) so it won't look that bad when it's finished but I will try it on my own (no art director).
The examples are helpful and I really like the style of your/sPIKe's last picture (http://quiksave.net/wp-content/uploads/2008/05/emag_lostwinds_041508_5_47695.jpg) and the style of Trine which looks really great, too (I'm not much of a fan of cartoon style like in Wind Waker). The detail and the visuals of Trine are unreachable of course and I cannot create three dimensional backgrounds as that would consume too much time, my last attempt 2 years ago failed because of this. As a result I will use 2D backgrounds with parallax scrolling (that is already implemented).
I started to learn Wings 3D some days ago and I will create a new main character, this one is 6 years old. Thanks again for the feedback.