First cruise with Princess - Embarkation question

Our first cruise with Princess is coming up this week and we just received the notice from Princess that our embarkation window starts at 12:00pm. Is this a hard rule? What will happen if we arrive to the terminal early? Will we wait until our boarding time or will we be allowed to board early?

Also, we are hoping to book a few dining reservations that were full and the Sanctuary and this time slot seems late to be able to get that done. Thoughts?

We’ve not been on a cruise line that enforces embarkation timing so this is new to us. We usually sail HAL or Celebrity out of FLL and once out of Anchorage.

Thank you.