Is this your first
Disney cruise? I ask because I haven't been on a Disney cruise and it might be a different de-boarding experience but on the cruises I have experienced the only ways to "get off early" are medical issues or tickets for e a r l y plane flights. Otherwise passengers had to wait for their group's scheduled time. (Not "group" as in touring group but rather by category such as alphabetical last name, cabin assignment, etc) Since we are "S" we were near the end of the list on our last cruise and spent a couple of hours between the time we were to be out of our room and when we were allowed to get off the ship.
There is also the consideration of ALL the luggage being picked up and taken ashore, categorized, sorted and available for pick up. Since we waited so long on the ship we didn't have to wait for luggage but if you get off early you might. Even once we were off the ship it was probably at least a half hour before we were able to get to the car to leave the port.
So...my suggestion is to plan a separate trip for IOA.
However, buying tickets for IOA before your trip and having them with you is a possibility. That way you can make a game time decision on whether or not you want to go for it. If you don't activate the tickets by entering the park they will stay good for your next trip and will encourage you to actually come back for that trip.