Willow Springs, near Reading used to allow season pass holders to dive all winter by simply calling ahead to get the code for the gate. I have not been there in 10 years or more, so not sure if htey still do this or if they are even still in business at all.
Also, the rivers are free and open all year as long as you can get to a boat launch or other access point. We have been known to dive in the Delaware and Lehigh rivers in the off season. I would imagine you could do the Susquehanna this way as well.
Dutch Springs does have 2 "instructor weekends" in the winter, one in Jan and one in Feb, during which you can have an instructor sign up and add you and others to their "list" to get in and dive on that designated Saturday and Sunday. This of course requires you to know an instructore who is willing to dive mid-winter and available on the specified dates.
Best of luck