It's a horrible problem.
So many times I see Vinegar recommended and if the water spots are actually mineral deposits on the surface then "sometimes" this will work.
If the spots are actually crater etchings in the paint, that is "whatever" was in the water was corrosive enough to EAT into and ETCH the paint then vinegar isn't going to do a thing.
If you have actual etchings, then the only way to remove the spots or crater etchings is to abrade the surface until the surfaces is level with the lowest depths of the crater etchings. This means removing some paint.
The deeper the crater etchings.. the more paint you have to remove.
The most important thing a person can do is to find out what caused the problem and then try to avoid it into the future because you cannot continue to compound a thin, hard factory finish as you'll go through the clear and expose the basecoat.
I point this out because I helped a guy once that got crater etchings from a sprinkler that went off at 4:00am every morning where he parks his car at a Condo and after he removed the spots he continued to park there.
Like dsottum suggested, you can try the Surbuf pads with the M105 and work a smaller area at one time so you remove more material.
IF that doesn't work then an aggressive wool pad on a rotary buffer with an aggressive compound will eventually remove enough material to level the surface.
I would try to avoid sanding unless you're going to do it or you know someone that knows how and is good at "dampsanding" using a very high quality sanding disc like the Meguiar's Unigrit sanding and finishing discs.
Keep in mind that it's easy to sand paint, that's putting scratches into the paint... the tricky part is getting them 100% out of the paint.
I feel for you...
