I need some advice on Water Resistant, Long-Term camera bags?

I plan on traveling for the next few years throughout the US and Canada, and I need a camera bag that's going to get me through it all. I'll be camping, hiking, swimming, kayaking, spelunking, etc.

Size: I'm a small person (5'3 at 120 lbs) so one of my biggest concerns with shopping for these bags is getting my spine crushed. I have a Nikon camera, 4 lenses, and a tripod. I would very much like to be able to fit a Macbook in the bag when needed, and other things like snacks/sunblock. But I can't deal with a behemoth bag. I'd like something with good structure so it doesn't hurt my back while hiking.

Functionality: I need the best in water resistant materials. I'd rather pay $400 for a bag once than replace thousands of dollars worth of gear. I'm not careless with my gear in the least bit, but I don't want to take chances. I love how the Lowepro Dryzone 200 even FLOATS, but that goes back to my first concern of being too much of a burden to carry. I need something that will last and doesn't scream "I'm full of $$$" (another reason why I lost interest in Lowepro Dryzone 200).

The one I'm currently looking at is Whistler BP 350 AW by Lowepro.
http://store.lowepro.com/whistler-bp-350-aw
It looks pretty good (size-wise), but I'm not sure it's as watertight as I need…

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