I use a dry bag (tested periodically in the bathtub) for my sleeping bag and another for my insulating clothing. The alternative is to use a pack liner. Nothing else will keep your insulation dry during a really heavy rain or if you slip and fall during a stream ford.

If you're using a bladder, leakage won't affect the dry bags. If you're using a pack liner, just keep the bladder outside your pack liner!

Personally, I won't use a hydration pack. Trying to suck water out of those hoses makes my jaws ache and becomes excruciatingly painful!