We plan a picnic at the park in August for about60 people. Should We reserve the space or we can stay at the public space first come first serve near the water for swimming and BBQ?

Check the website

Most times there are no additional charges for a picnic in the park unless you are utilizing a designated area such as a pavilion or a table and grill. It is best to text their specific location by visiting their website: http://dnr.maryland.gov/publiclands/Pages/southern/sandypoint.aspx If there isn't any information there then try and call the phone number provided on the Maryland. Gov site to ask specifically about the Sandy Point State Park.

I believe reservations only are for pavilions (areas with a shelter). Each shelter area has one or more BBQ grills. The swimming areas are probably open to everyone. However, I don't know for sure (I've never reserved a space at this park), so you'd be better off calling the office.