It all depends on what you're doing.
The Port itself opens at 10:30 for check-in.
They usually begin boarding the ship itself around 12 noon.
The line to board the ship usually begins forming around 11-11:30.
If you want to make palo/spa reservations, without having to use fallback choices, you want to be at the head of the line.
12:30 is a perfect time if you just want to just walk on to the ship, and have no concerns about making any special reservations.