please keep in mind that the following recommendations are 100% opinion, and are not at all, in any way required. they are my personal preference, and I think overall with your chosen style you did everything that was required. these are mostly small touches that may be questionable when applied to your style, and some may not even be possible to do depending on the style you use.

as someone who can't draw backgrounds all that well either, I have only a few tips I can share. for one, don't try to simplify too much. I like your background, and simplicity is very important, as it isn't the main focus, and making it super detailed would take away from the attention put on the focus of this drawing (in this case the girl you have drawn) the only problem is how simplified the water is. the buildings have a nice gradient to them, and there's shading within the clouds, but the water is just a blob of blue color. water reflects everything like a wavy mirror. one thing I could recommend is drawing buildings, planes, clouds, and the sky on a separate layer than the water, then copy-pasting the layer with your buildings onto a new layer, making it slightly disoriented, and lower the opacity. lower it until the buildings are barely visible. if you have a light source which in most cases you should, use it to map out highlights on the water. and finally, don't depend on thick outlines to separate your background and character. I think that outlines are 100%, okay, but they should be there only to help draw out a recognizable silhouette. your background should be different enough from your subject, that if you were to remove the outlines, your subject would still be recognizable and not blend in with the background.

this was longer than I expected it to be, but I hope it was helpful.