Hi Rebecca,
I've never used the "waterproof cement"- I'm not sure what the waterproofing agent is, but regular ferrocement is already waterproofed (especially when you finish it with a layer of cement wash- water + straight cement).
Here's a clip from Ferrocement Educational Network about freezing:
"...In some cold areas of the country people will leave water in their swimming pool over the winter knowing that it will freeze, at least for some inches or feet. You see,
If they take the water out of the pool the ground will freeze on the outside and with no ice inside to resist it will crack the concrete pool. To be sure that doesn't happen they will throw a length of telephone pole or other large wooden object in the water before freezing starts and this relieves the stresses
enough from the ice inside the pool that the pool doesn't crack..." I'm not exactly sure how the pole relieves tension- I've never had anything that big (that I would throw a telephone pole in it), but it's worth a try for the winter.
I would definitely lean toward keeping water inside it during the winter to balance the ground freeze force from underneath. Also,
FOR SURE, I would use stronger metal reinforment than chicken wire if you are looking for durability! Hardware cloth, flexible items like fiberglass mesh, metal diamond lath, pencil rebar, etc. The cement binds to the Metal Armature, and the structure is only as strong as the Armature. They can definitely withstand freezing if done right.
Best to you with your project!