Plan to get there a little earlier. They normally start boarding at noon. The reason for that 2 hour rule is the government requires a final manafest of all passengers onboard one hour before the ship can sail.
If you arrive a little after 2 pm no big deal. If you arrive after 3 pm you may face issues unless there are a number of passegers with NCL air delayed as well. If the last passenger is onboard at 4pm the ship can not sail until 5pm. They are not going to hold up all the other passengers for just you.