I just picked up a Baggalini messenger bag from amazon.ca for less than $40 CDN and am excited to try it on my next WDW trip. I had an old MEC small field bag (like REI) and it was a bit smaller, really the perfect sz, but no mesh bottle holder. I used to carry a small plastic H2O bottle with a rubber strap holder dangling from the side. Not ideal as it would bounce against my leg if I was walking quickly.
@maxiesmom does the Baggalini messenger bag fit into the mesh pockets on the rides? If not, its not a huge thing as I can put a foot through the strap if necessary.