It does sound like a perfect pond site - except for the portion of the construction phase where the contractor is trying to drive a dozer on wet clay.
However, we did not ask some important questions.
How large is the proposed pond? Embankment type (with a dam) or an excavation type?
Finally, how is the contractor's bid structured? If he is doing the work for a fixed price, then it doesn't matter to you if he has to fight the weather during the job. It is probably a good deal for both of you to fit this work in when the season makes his other contract work slow down.
However, if he is charging by hourly or day rates for his equipment, then the weather hardships come out of your pocket.
P.S. Do you have a lower location on the property to allow for drainage of the groundwater from the pond site during construction? This can be either by a cut trench or by pumping. The hole filling with water will make excavation, moving the spoils, and compaction more difficult to achieve.
However, if the groundwater level is at roughly the same level during all four seasons, then you will have that battle regardless of the time when you build the pond.
The upside is that you will always have a full pond, which will give you much less stress when you are managing your fish!
