Can you swim in the lake

Swimming is NOT permitted in the lake. https://www.dcnr.pa.gov/StateParks/FindAPark/MarshCreekStatePark/Pages/Swimming.aspx

Yes you can.. its just a lot of rocks on the floor of the lake so bring water shoes

Yes

Yes, but it's very limited at least where we went by the boat rental/snack bar. It was fun for little kids and the water was very warm. There are several places that are more secluded but still limited swimming. If you go wear water shoes! It was pretty Rocky on the bottom.

Yes there is a beach ⛱

Not on the deep areas