I have heard that if you want to water proof/resist, stay away from water based products. After you dye it, I'd buff the living hell out of it to ensure excess dye is off and then put some non-water based finish on it. Sno-seal works pretty well at water repellency, you can also make your own with bee's wax and neatsfoot oil. I don't think there is a single one time treatment though, everything will need retreating after time. That time may be years, but it'll need it at some point.
Please let us know what you opt for. I'd like to know myself.