That's so weird, I only have heard of water marks getting on the handles, not the bag itself. Do you have any photos?
Yeh, the handles too, not the actual monogram but all the pale leather bits, when I had my new bucket years ago, I was caught in the rain and the leather round the bottom had watermarks after drying it out (just a little, but still...) and so did the handles. I don't use any of my monos in the rain now, damier and epis are ok though.
Someone suggest to use the Mono bag until the leather trims get darker then it won't be so noticeable but never use the bag long enough for it to go dark, the bag is still like new but with watermarks.
Someone said leave the bag in contact with sunlight and the leather would darken. I found the older LV mono canvas leather trims darkens quicker but the new ones doesn't.