" how does one keep their shoes from filling up, and in turn, your feet going full hypothermic?"
I don't think you can. You either have to:
- wear tall shoes or booties and long pants combo, or
- duct tape booties to your bare shins.
I've tried Velotoze, but they shred too easily.
I tend to wear a winter riding shoe with wool socks, as wool absorbs less water than cotton and still retains some warmth, when wet.