If you are relying on primarily the water table to fill the pond, I think that maybe changes things a lot.
When everyone was bringing up compaction, and the possibilities of poor compaction due to cold temperatures, that line of thinking was in order to keep water IN the pond. The same line of thinking would not allow water enter the pond meaning that if you seal something one way, it's sealed the other way too.
If it's as simple as digging a hole that will fill with water, maybe it can be done any time of the year.
