On a NCL ship, you need only make reservations for dinner at the speciality restaurants, pay or free.
For the two main restaurants and the cafeteria, just walk right in, or wait in line.
On at sea days, the main restaurants can have long lines if you're later than 6 pm or so. Many rush to dine when the restaurants first open, around 5:30 pm on most at sea nights. The restaurants will fill up by 6:30 pm, but since it takes 90 minutes to eat 5 course meals or more, it'll be 7:00 pm before the very first diners leave.
So there's a 30 minute to 45 minute period of time when no diners are being seated. Either show up early, and wait to dine at 7 pm or later if you wish to avoid waiting for a long time for a table to open.
Good luck on your cruise, August is the start of the hurricane season, and itineraries can change. booking a cruise at this time of year one must be willing to face changes as the weather changes if you wish to have a great cruise.
I sincerly believe that those who report having terrible cruises are looking for things to go wrong, and overreact to things that go wrong. Just about every cruise line board has terrible reviews from cruisers affected by Hurricane Wilma. Hey, it is not the cruise lines fault.