If you freeze the Utrics. solid before they have formed turions, I'm virtually certain the plants will be dead the next time they thaw. I've never seen otherwise, although it's interesting what Tamlin says about his turions surviving it. I believe in nature they sink below the ice level.
I've successfully kept U. macrorhiza over winter, along with others. The main things to do is keep the temperature above 18 deg. C (25-30 is best) and avoid algae!!! Algae is the biggest menace to the plants, and in my opinion why they are hard to grow long term indoors. The ph should be on the acid side, but I've found the plants tolerant to 7.5. Provide good lighting.
Another thing I strongly suggest is that you divide the batch you have and place some in a jar at the window. The draftier the window the better. (you want the jar cool but well illuminated by sunshine) If you're lucky, and you very well may be, these plants will produce turions, which can then store in the fridge.
Good luck!
Chris