If those two pools in the first image are supposed to have bevelled rims like a modern shower tray, you could use the technique I used for my shower tray.

Cut out the overall pool shape as a separate drawing (you'll import it again later). Take this new pool-only drawing and select a region inside it. Invert your selection, so you have the rim selected, then hit your bevel command. That should give you an internally bevelled rim. Import your rimmed pool back into the original drawing.

I probably have no more 'flying hours' on Gimp than you, so there may be a better way, but it should work for you.

I can't believe it - I'm actually giving a tutorial on Gimp!