I made the attached image to help explain some of my suggestions.
I think you should make two gradients, one for the sky and one for the lake to define the horizon. Also you could add a reflection in the water for the trees. and where the tree line meets the water could use a little attention to look more clean and fluid.
Just out of curiousity...is that the type of boat the guy has? if so then no problem, but if not...it does makes him look cheap.