I really like the ArcTeryx Arro 16 backpack
http://www.altrec.com/arcteryx/arro-16-backpack?sku_id=312361?cm_mmc=Mercent-_-Froogle-_-Arc'teryx-_-36592-312361-B&mr:trackingCode=601BB294-E9D4-DD11-8F1F-0019B9C2BEFD&mr:referralID=NA
I put a Domke 4 or 2 lens insert inside and I am good to go.
I do not use the water pouch because I am afraid of leaks inside. That said, the bag is not 100% waterproof, but it does a great job for me. I also sometimes use a small fannypack type bag to hold a lens or cards while shooting, but the bag can easily be swung around and accessed.
Also, Tenba makes a slingbag that would work well for what you need and I think it's cheaper than this bag.
Tenba - Shootout: Small Photo Sling Bag