I'll be honest... the map ain't bad at all. You used colors that go well together and it is perfect size for an encounter which, is what I'll assume this map is for.
I could give you hundreds of ideas on how to improve it but... really, it seems to me you need just play around a bit more. You have a solid foundation and just need practice learning your preferred program. I could arrange many encounters just given the map you provided and, well, if you look around this place enough, you'll find many ideas that would help give a more realistic feel if that is what you are looking for.
It's hard to just up and tell someone what to do with a map when the map maker hasn't stated the purpose of the map.
Did you create it just for fun? Is it going to be used in a campaign? What purpose will the map serve? I could make passable maps all day long but... a true work of art would be somthing that has a purpose.
Tell us what you intend this map to be used for and we can help you tweak it as you need but... it's hard to comment on a project that might not have any purpose.
Answer these questions and I'd be happy to state my opinion on the map. As it stands, I'd darken the grass area.... put some sort of shadow under the dock, and maybe lighten up water around shoreline....
The dock itself could use work as it just doesn't flow into the scene but... I'm sure you can work that out... try Satin style effects and drop shadow to make it look more realistic.
The water looks great and one can tell what the grassy areas are but... well, if this is yer first map with Gimp... methinks yer well on way to a good future. I've seen some rough stuff come out of newbs and... yers' manages to stand above all the shiznik.
Keep practicing and feel free to PM or email me if ye could use any help.