Hi Jo, here's a guide on how to remove water stains from marble: https://homeguides.sfgate.com/remove-water-spots-marble-countertops-90254.html
A great question and here is some information for you! Good luck.
Here's another guide Jo
I use WD40 on flag stone, tile and it works great. I spray WD40 on the area, leave it overnight and wipe it dry the next day. I've not used it on marble but it wouldn't hurt to try.
A slurry of hydrogen peroxide and baking soda. Cover the stain, let it dry, wipe away.
I clean mine with baking soda and water. Make a paste, let it sit for a few hours and clean it.