Can you tell with your fingers if the stain is above the paint (water deposit) or below the paint (etching)?

If it's above the paint, try the clay bar as recommended. If you're not sure on how to use a clay bar, try Meguiar's X Scratch remover. Use a light application with a microfiber towel or an old cotton t-shirt.

If it's below the paint line, the X Scratch remover will remove it if the etching is superficial.

If it's a deep etching (enoough to catch a fingernail), then a detailer will have to polish it out with a orbital polisher. Elbow grease alone won't be enough to smooth out the surface.

After removing the surface defects, you'll need to apply some paint protection like wax and sealant. Might as well do the entire vehicle. See the detailing section on recommended ways of maintaining the finish of your X5.