In my experience with non
Disney cruise lines, they want you off the ship pretty quickly once customs is cleared. I've never been off later than 9am unless we were taking our time due to a later flight.
As for the parks, we just came back last night and the parks were crowded but very manageable over the weekend with express pass (even with the early closings for Grad nights). The HP Diagon Alley area was pretty empty by 8pm on Sunday (9pm park close). We rode Gringotts 2X right before 9, pretty much walked right on though the stand by sign said 10 minutes. Everyone seems to hit that area in the morning and I think it's much better after dark with the dragon and all... plus it gets very hot and crowded over there during the day.
As for IOA - I would want to be there during the hottest part of the day to cool off on the water rides. That is what we did both days of our trip - made our way over there after lunch and stayed wet and cool for a few hours before going back to the hotel to freshen up and heading back out for dinner and more park time. The longest wait we had was for Forbidden Journey when we ended up behind the pack at early entry because we had to stop for Cinnabon and Starbucks!!!
If I had 1 day only and express pass I would start at US and do all of the non HP stuff that I was most interested in then have lunch at Finnegans before taking the train over to IOA and go to HP/Hogsmeade/Hogwarts then make your way out from there hitting the water rides during the hottest part of the day. Go back to US that night for Diagon Alley and redo the best rides like Mummy and Minions on the way to London.
Also, if you run into long stand by times at Gringotts or Forbidden Journey, there is always the single rider line. Just be sure to also take the tour of Hogwarts either before or after single rider so you can see it all.