They work well drying the water or excess solution as well.
The watermarks could be from the softener reacting with the paint, in which case, it won't come off, you will have to touch up with more paint.
It could be the glue from the decal, or a little contamination from the brush or whatever you used to apply the decals.
A flat over spray can hide faint watermarks.
You can also attempt to blend it in with any weathering you are doing.