Interesting concept, but like you said, it's going to need a bit of work.
Your colors don't look very natural. Do a Google image search for images of rivers or fjords and mountains to get a better idea of what such a scene would look like. Shadows are generally darker, less saturated and more blueish.
The green is too saturated, the mountains should fade with distance, and the cliff should be more uniform. Shouldn't there be snow up there? Looks like it's pretty high up judging by the perspective (the snow on the mountains follow the same horizontal line).
The river looks small. I don't think the rapids would be that pronounced from that distance. Both the lake and river should be darker since the mountains would be reflected in them to some degree. At the very least, they should be the same color as the sky. Water by itself has little color; it reflects what's around it.
The penguins do look flat. Try and find some round shapes and look at how shadow and light follow their form. The penguins should also cast a shadow.
I hope that's helpful. Looking forward to seeing your edits.
