I do like a happy ending
Glad to have been of some help, though I think it was a combination of answers that solved the problem.
I know nothing about layers.
The principle of overwriting the Route and Session after any change of an object appears to be the key, whether it's done via Quickdrive or regular Save.
I've not tried your method of changing the water colour. I must try that one, although I guess that there is a risk of then having a water colour that's not wanted.
I'll probably stick to simply rotating a small object (usually a small grass tuft). As little as one degree of rotation is all that is required, so it has little impact on the scenery.
Cheers
Casper
(Still in the stone-age!)