Thank you all for the tips and suggestions!

I'll try to play around some with the colour of the water, hope I can get something a little less vivid...

As to the rivers, those are still little more than sketches, done with the "path" tool in PS, i wanted to work on them but then got carried away naming stuff :-)
And I just realized that it really looks like the river up north flows across a mountain range, but that's just flat tundra there. But I'll tweak the colours a little to make that clearer.

Another question which arose when I took another look at the place names, what's your opinion on those? I am quite satisfied with the made-up ones, but the english names started to seem a little weird when I thought about them again. Has anyone got any good tips on naming places?

@jbgibson: I'm not sure whether I completely understand your suggestion on how to do the rivers, would you mind to elaborate on that?

@midgardsormr: interesting obeservation about the mountain texture, have you got an idea on how to improve that? cause i never really liked it but everything i tried made it worse, so i decided to leave it at that.