We went on our first cruise when my youngest was 14 months old and we spent a little bit of time in the Mickey's Ear. It is small (maybe 5ft diameter round) and the water motion resembles a big toliet (I thought that was fitting), but it is a good alternative to going in the big pool for that age.
The thing that bothered me was the occational bigger kid jumping into the Mickey Ear and running around, splashing without any regard to the little ones. I know that kids will be kids (I have four of em) and they don't realize in their excitement when they knock over a baby, kick them, or splash them in the face. I don't remember if it was posted that the Mickey Ear is for only small children and their parents, but IMO it should be. I would hope that parents of the bigger kids would make an effort to keep their kids from jumping and being wild in the kiddie pool for the safety of the little ones.
Now the my baby is just over 2, I hope she won't be upset about not being about to go with her siblings in the bigger pool. That may be a bit of an issue (tantrum) so I don't plan to use the pool with her all that much. We have a pool at home and I'm not crazy about crowded public pools, especially one full of 4-12 year olds. Yikes!
BTW, I rarely use swim diapers in our own pool and my daughter has never had a BM in the pool. BMs, for her, require privacy and concentration, and I can't imagine her doing that while in the pool, KWIM? Peeing, on the other hand, IMO, is not limited to toddlers. I imagine there are many older kids (and even adults) who pee in the pool. Let's face it, it happens more than we would like to think!
Lara