My favorite messenger bags are these - BaileyWorks Bags: Messenger bags and more, made in the USA - I've have one that's about 15 years old now and still in daily use and another, smaller one that I added about 6 years ago for work. The rear zippered pocked would make a great waterproof ipad storage area and the rest of the bag has all sorts of space to store/tote whatever you might need.
http://www.baileyworks.com/Messenger+Bags/253+Courier/options/#
That's the closest model to the one that I currently use for work.