All beaches in the DR are open to the public...access to the beach may be another story...
Beaches up to 60 meters from the high water line are public--if you can get to it without crossing private property.
There is a door that has been provided so that locals can get to the beach next to the Casa Marina beach. Isn't that still open?
If you run across the beach that Casa Marina would like you to think belongs to it, pay no attention to the guard who tries to chase you off. Run close to the waters edge so you are well within the 60 meters. You have a legal right to be there. Just don't try to leave the beach by going through hotel property.
