I now dive year round, but when the quarry closes in October and the weather in April would often be cruddy, there are your six months.
Only quarries close to here are over the border. Charters on the Lake begin (sometimes) in May, but normally June. There is no such thing as walk ons here. All trips are booked by shops, and mostly out of the area ones. All of this ends by October 1. Folks are pretty lame here with local diving...