In GIMP you can bend text by making a path that follows a river on either side, then typing a text and using the "text along path" function - this will create a text-path that runs along the river.
2nd: This map would definitely benefit from some coloring, even just adding shades of gray. Your rivers look almost indistinguishable from coastlines, while the land and the sea/lake areas are filled with the same shade. This makes it difficult to discern at a glance where is land and where is water. If the sea (and the rivers) were colored blue, that already would make a huge difference.