You shouldn't have to move your landmasses around at all, if your ocean is on a layer by itself. In photoshop you can enlarge your canvas (not image, which will just magnify everything, but canvas, which will enlarge the actual area of you piece of art) and I'm guessing there's a similar function on Gimp, though I don't know what it's called.

If that's the case, you can just enlarge your canvas, leave your land layers alone, delete your ocean layer, and redo it so it fills up your new, larger work area.

If you've already merged all your layers, or you don't have an 'expand canvas' function, well... consider my original comment a suggestion for next time.