Getting on board was fine for my wife and me. We followed advice from previous reviews to try to show up early and be on one of the early buses from the San Diego cruise terminal to Ensenada. Calling the Celebrity phone line, the call center agent said check-in would start 12 noon. We showed up a little after 10:30 am and check-in had started already. We eventually boarded the ship by 2:30.
Waiting on the bus in the Ensenada parking lot took 30 minutes since they were queuing buses and only loading people onto the ship 2 buses at a time. From talking with other passengers, some people did not board until after dinner.
We strategically had our room located between the aft section and the glass elevators since the food locations are on the aft and the entertainment venues are mostly on the bow. For C3, the highest we could get was Deck 10. (Note that the cabin number 1257 is not on Deck 12.) Relatively speaking, Disney cruise cabins are usually larger. Generally the cabin attendant is very attentive and you'll never be short of towels, ice, or turn-down service chocolate.