The immediate effect is that the water appears to be running down hill. This happens to me quite a lot! The tree branch upper L is intrusive; you may be able to remove using Clone Tool.
that's gorgeous!
However, the water and the sky seem to drift into each other with the colour. I agree the tree branch is out of place in the photograph. Overall you did a great first attempt.
Call me crazy, but the reflection doesn't seem to line up right. If you are going to add a reflection (and i agree, it must be to glass-like still water), make sure it's proportioned right. For example, the horizon should really cut the two mountains in half. And it appears that the 'reflection' mountain is too short. Does that make any sense to anyone?