There are two buffet restaurants, one at the Divi and one at the Tamarijn. You may dine at either one and neither requires a reservation for breakfast, lunch or dinner.
The ala carte restaurants http://www.tamarijnaruba.com/restaurants/index.php
you really do need a reservation for dinner. On an off chance you might be able to get in, but no way would I ever plan on it. Our friends tried last minute to get in one night at the Italian restaurant, Paparrazi and were told no. We were able to add the four of them to our table of three.