You should check the opening times for Islands of Adventure. I think you will run into major crowds and VERY long lines if you try to do Harry Potter that day IF you don't get there right when the gates open. Even if you get there before opening time, all those folks who stayed on property are allowed in 1 hour early. By the time the "regular" folks get in, it is completely mobbed. When we were there over spring break, the lines for Forbidden Journey (castle ride) were HOURS long!
Would it be possible for you to spend 1 night at one of the Universal hotels the day you disembark? If you stay onsite, they give you a "front of the line" pass + you can get the HP World early entry. Upon disemarking, you could check into your hotel, get your front of line pass, and go over to Universal Studios. Save Islands of Adventure for the next day so that you can get there EARLY. That would be my suggestion. We were there for spring break and stayed at Royal Pacific for just 1 night - just so we could get HP early entry and the front of the line pass. Or if you don't want to pay for 2 park days, then just hang out at their pool the first day - nice pool and very relaxing. Note that you cannot use the front of line pass on the Forbidden Journey. Thats why you have to get there before the gates open so that you can ride it first thing. Otherwise, lines are aweful.
Good luck! We LOVED HP World!!
Jodie