I think you could build a static pressure filtration device that would work as well as, or better than, the osmotic pressure system described here. A constant positive pressure across a desalination membrane should work. I can imagine a bladder system that sits in the bottom of the liferaft. You fill it with a few liters of seawater, then cap it and lay on it for a couple hours. The gravity load should be sufficient to drive the fresh water through the membrane. Then instead of sugar water, you just have plain water.