Did this last January with my 2 1/2 yr old. She did fabulous! We asked to talk to the Oceanears Club director, and explained that my daughter was in preschool 5 days a week, with me, and that she was COMPLETELY potty trained (they do not allow ANY pullups). The director "interviewed" her for lack of a better way to put it. She had her go through a couple of simple 1 and 2 step instructions. She said that was the biggie...since they take them from place to place, she had to be able to follow directions and stay with the group. And, of course, she had to be completely potty trained. She said she would have to contact me later in my stateroom. I guess they had to know how many they would have that week to know if they could take a trial. She called later, and said that they would take her on a "trial" basis. She would be allowed 3 potty accidents, and then wouldn't be allowed back. Happy to report that she only had 1 accident, and they never had to page us because she was upset. They told us at the end of the cruise that it is unusual for a 2 1/2 yr old to stay the whole time, but DD did great.
Overall, I'd say give it a shot. The worst they can tell you is no, and then you just go and reserve a spot for your child at flounders.
I will say this...we did everything "discreetly". Meaning, we didn't flaunt the fact that they allowed her in. We asked to talk to the director privately, and we didn't make a big deal about it with other passengers or counselors. Attitude seemed to make a difference.
It WAS cool to see my little pumpkin up on stage at the end of our cruise in "Discover the Magic"!
Hope you have a wonderful time.