We were in the Haven a few weeks ago. We booked all of our diner reservations 125 days before the cruise. You can book at 12:00am eastern time on that date. We had no problem booking dates and times we wanted.
While on the ship we heard that Bayamo by Ocean Blue was very good. We didn't have a reservation for the restaurant and it was booked up. We asked our Concierge if he could get us in and he got us a 7:00pm table.
Also, there was an excursion we wanted to do, however it was sold out. We checked with our Concierge and he booked us. Haven life is good!