If it really bothers you I would suggest trying Saphir Reno. I'm not a big polish guy if I can avoid it, and I try to use as little additional substances as possible but reno has worked wonders for me on both calf and shell, Im somewhat amazed by it, and it leaves leather (particularly calfskin in my experience) with a very nice look and feel. I set a shell wallet down the other day on my kitchen counter, it picked up a similar circular water mark, although I think it was from a frozen blueberry. I put a tiny drop of reno on, and it looks better than new. Good luck
Polish is the only real way to correct for color loss/changes in calfskin. If you aren't willing to use polish, at least occasionally, then a year from now a small water spot will be the least of your worries. Renovateur can only be expected to do so much. It may keep the leather in good condition (assuming you don't over use it), and keep a healthy looking "glow", but eventually it will remove color, and it won't hide all of inevitable scuffs and scratches. Of course, if you are actually after a "well aged" looking shoe, then disregard this. I'm not sure that's the case though, if you are very bothered by a few small water spots.
Shell is a horse of another color.