Tonih, There will be a dog park opening in St Mary's county in March. It will be in the John Lancaster Park on Willows Rd. I really don't know much about that area so I can't give you more specific directions, I only know that that's where the dog park is.
There is another dog park that's either in northern St Mary's or it might be Charles county, I'm not sure. It's in a park called Turkey Run or something like that. In Charles County, there is a dog park in White Plains Regional Park. That is in the same general area as the Waldorf mall.
For anyone in Calvert County, there is a dog park on Gray's Road which is in the Prince Frederick area. There is also going to be one in the northern part of the county in the Dunkirk Park and eventually I think there is going to be one in the southern part of the county also, down in Cove Point Park.