To solve the light position I would put a fake "mangrove" or other type of fake tree in the back corner, up against the wall with roots going into the water. Let the tree hang over the water halfway and put a light on one of the branches. Cover the sides of the light with leaves and such. I think that would look awesome. Everything would have to be fake and painted, but I think it would really add more character to the garden and would solve the light issue perfectly!
horrible rendition but just to show you what I mean:
