B-bags can take a beating with the water. Water will make the leather dark but if left to dry by itself, will return to normal with no marks. But since it is such an expensive bag, most of us (but not speaking for all) hate to take these bags out in wet weather. I personally don't like to take b-bags out in extreme weather although I have been caught with it in rainy weather without any consequences. I don't know about scotch guarde but Apple Guarde is highly recommended for protecting b-bags.
Apple Guarde can be purchased online: leatherstuffs.com
The other option is to bring another bag large enough to hold your b-bag (like a plastic shopping bag or tote bag) and put your b-bag in there when the weather gets pretty bad.
Even though water doesn't affect or ruin it especially when they're protected with LVM pre-treament or apple garde I'd leave it at home because you want to have fun and not always to worry about your bag. Have fun on your trip!!
CAUTION! Some Balenciaga leather colors will waterspot and only a professional can remove the stain. Leather protector sprays like AppleGuard or Kenneth Cole Leather Protector will seal the leather color...but I don't know if ScotchGuard is designed for leather fibers.