Unbuckle yourself! You can't get out unless you are free of that restricting strap.
Roll down your window if you are able to. If not, find something like a S.L.A.B. Belt buckle pin to break the driver or passenger window and break it (don't attempt to break the windshield).
Get those out of the window that are youngest first. Remove your body with your chest facing up, head-first out of the window. Push away from the sinking car
DO NOT try to retrieve anything from the car
Swim away from the vehicle and call 9-1-1 Emergency Services.