Its been a while since I detected there but they have some rules on when/where you can detect. Once the beach is closed for the season I believe its open to detect but one of their rules is no detecting within 15' of the waters edge, I cant recall if its different for the beach. Also, the rules state that you cannot use anything larger than a standard screw driver to retrieve targets. Like I said its been a while since I detected there, I'd suggest calling or stopping by the park office to see what the rules are. Last time I talked to them they were very nice and the guy was really interested in the hobby.
I detected the beach area in 2008, but only in the sand area. I stopped at the rangers office to ask permission. They asked if I knew the rules, and asked what area I wanted to detect. Originally I believe I emailed or looked up a phone directory for the park manager. I have detected cascade park a few times too, but haven't done very well there. I like Presque Isle, it is heavily detected but the park is huge, I prefer to go in the off season though.