I had my water table pond excavated last December in gray clay soil. It was filling before the excavator was finished and wasn't compacted. It hasn't rained much in the past month and a half and its only down about a foot. Maybe Maine clay is denser?
A true "water table" pond IS NOT clay sealed. It must be hydraulically connected to the groundwater supply.
Of course, there are exceptions. Such a when you have a spring that takes and gives water at a certain pool elevation, with a clay-lined pond basin below that elevation. (This would keep the pond holding some water after the spring dries up during the dry season.)
