Sagrada Familia

If you can get an early enough time slot, I would definitely choose the second day. Many, many people are totally brain-dead after flying overnight to Europe. It's a combination of jetlag and sleep-deprivation. I would not want to go to what is arguably Barcelona's top sight in that condition. Instead, I'd plan some walking around outdoors on the arrival day. Perhaps the Barri Gotic (medieval).

On the other hand, for me, it would be better not to see it and worry about making the boat at the same time. Therefore, I'd go that first day--you need to stay awake anyway--.I'm glad you're going to see it; I think you'll be glad too--so much of its beauty is just "being" in the space. If you read ahead a bit you can go right to things that piqued your interest in the first place, eg. certain carvings, or geometries, etc.

We also took a cruise in October 2017 that departed from Barcelona. Like you, we arrived the day before. We booked our Sagrada Familia tickets for the morning that our cruise departed. Our entrance time was 10:45-11. Our cruise ship was open for boarding from noon till 4 in the afternoon. Our plan was to get to the cruise port between 2 and 3 so that we had a little cushion in case of unforeseen errors or travel mix-ups. We had no trouble seeing the Sagrada for several hours that morning, even eating a few tapas before taking the metro toward the cruise port. The Sagrada opens at 9 am in October. Even if you have tickets for right when it opens, that may not be enough time to see it adequately if you have to be on the ship at 12:30.

I think it also depends on how quickly you tour things. If you already need to be on the boat by 12:30 the following day, you might be rushing yourself to do the tour, get back to your hotel to get your bags and then get to your boat on time. My husband and I tend to be on the "quick" side of touring compared to most, and we even took about two hours to explore the full site (using the audio guide and one of the tour climbs).