Mercury does not have a specialty dining room. They do set up an alternate dining (casual) area either in the buffet area or on the deck around the aft pool. As I recall
you need to make an advanced reservation and there is a limited menu. I do not recall that there was an extra charge. This arrangement seemed to be utilized by those wishing to skip the formal attire in the dining room and for families where the dining room was not working out for the kids. Room service will serve you anything on the dinner menu in your room during regular dinner hours.