have you looked into the Ortlieb's products line ? They build very solid and absolutely waterproof panniers/backpacks, but those usually don't have inside compartments.
I personally use a Backroller Classic pannier combined with a carrying system.
Not as comfortable as a "natural born backpack" but as long as you keep the weight under 6-7 kg you can use it extensively as day pack. I've climbed a few mountains with it.

p.s.: this one has an inside removable pocket