How the restaurants work is really quite amazing. They serve 100's of people at one time so everything has to go in order.
It does not work well to show up late for your dinner because the kitchen is timed so they can feed everyone at the same time. Also if your seated with another family it is hard for them to start just part of the table.
The tables are assigned, so it's not possible just to go to the later seating.
So suggestions are:
Tell your Head Server you will be late that night and he may be able to get you a table at the later seating if there is one available. On a
Disney cruise you're serving team follows you to each restaurant every night. You may not have the same serving team if they move you to the later seating.
The buffet restaurant on the pool deck serves a sit down dinner every night except the first and last night. They serve a smaller version of what is served in the dining rooms. Plus a little more
The fast food restaurants on the pool deck will be open.
Also room service is available at no extra charge but you should
give a little tip.
Again talk with you servers or head server they want you to enjoy every meal.