What zone are you in? If you are above Zone 6, you have to anticipate
freeze problems. I am in Zone 5, and my 4X6 3.5 deep 3/4 inch plywood
pond has 3.5 inches of foam board insulation betwen the inner and
outer walls, and I put a 100 watt aquarium heater in the water in the
winter and cover it with a two inch thick foam insulation board. It
never goes under 40 degrees, even when it's below zero outside. I also
have foam board under the liner, but I don't know if that helps or
hurts-- maybe the earth heat would keep the pond warmer.

I'm planning to make a 8X8 four feet deep pond this fall, same
construction, regular 4X8 CDX plywood and treated horizontal studs
screwed together at the ends and to the plywood, and rubber liner.
Just like you would make a concrete form. I am thinking of digging a
hole in middle under the pond so it would be five or six feet deep in
middle. The liner does bunch up in the corners but that's only a minor
appearnace problem, not a functional problem.

My pond is not on my deck, it's next to my deck, so I can look down
into the pond from the deck. Or jump in if the spirit moves me.