It is so hard to guess what your son will be like in a year and a half. I totally understand your worry and hesitation.
That said, my son is 4 and has been on 3 Disney cruises. On the first one he was about 21 months, second one he was just over 2, and then 3.5 last year. He had a wonderful time and so did we. We had to deal with Flounder's Reef Nursery the first two cruises, they are great, but it is a lot harder to work around than the club (because they are only open at certain times).
They start to slow things down in the evenings in the club, and the kids go to sleep as they get tired. We left my son down there until at least 10 or 11 every night and he would be sleeping when we got there, they had little nap mats. They sort of kept one area of the club more quiet and dark, and then some kids were up partying into the wee hours.
My son is usually so tired on vacation that he doesn't really fight it. And my DH would carry him back to the cabin and he wouldn't even wake up. LOL
It is hard to say if your child will potty train by then, and it will be really hard for you not to stress about it as it gets closer. Try not to. (I know, easier said than done.) My son was not totally potty trained at 3, but was OK enough to last short stretches in the club without an accident. We would check on him often and remind him to go potty. The CM are really good about this, too. They understand that age is new to the potty thing and they do lots of potty breaks.
I think it is a great age to cruise, your son will have such a fun time. At that age the simplest things will be so cool, he will enjoy just looking out the window at the water, going up in the elevator and looking down into the atrium, ice cream at any time of the day, and splashing in the pool. And you should be able to get some time with your husband while your son is in the club. Even my SUPER shy son walked in and saw the pirate ship slide, and was like, "bye Mom"! It was harder for me to leave him, LOL!
HTH a bit!