My family had a 10:30am-11:00am check-in time. We showed up around 10:50am and the line was non-existent. Check-in was a breeze and we were on the ship in less than 20 minutes from the time we got to the port.
My cabin was 5303, which is considered a 6D Oceanview room. I booked the week before the cruise so there wasn't many options to choose from. This room is very loud. You will be able to hear everything from the Red Frog Pub and the Alchemy Bar. This is usually until about midnight, so if you go to bed early do not select a room in this area. This is my 7th cruise with Carnival and usually while you are in your room, you cannot hear the general broadcast announcements from the cruise director. However, there are no doors to block or muffle the sound from the common areas, therefore these general broadcast announcements can be heard in the room and are loud as hell. Good luck if you are trying to take a nap midday as it will be interrupted with these loud announcements about bingo sales or spa specials. Lastly, don't attempt to look out the window as you will not see anything. The window is super dirty and all you see is the white salt buildup on the windows.
Pros about the room: The bed is super soft (if you like it that way) and they have USB charging ports near the bed.