When we cruised in Oct the fountain play area for non potty trained children was strictly enforced. My 1 1/2 yo hated the fountain and wanted to be in the pool. She at the time was just starting to use the potty (I know this is young but everyone attributes it to her older sister). When we first went to the pool I put a swim diaper on her not because I thought she needed it but more because my husband said we brought them so use them. I ended up taking her back to the room, putting her on the potty, changing her swim suit and not putting on a swim diaper so I could take her in the pool.
At the same time there was another lady in the pool with a 3 y/o that they told she needed to take to the fountain because she had on a swim diaper. The childs mother said she was potty trained but she thought she needed to wear one to go in the pool. So if your child is trained don't use a swim pant or anything that looks like a swim diaper. Another woman had a swim bottom for her 2-3 y/o that looked like a diaper but was not she said and they made her also go to the fountain even though she stated that the child was trained. It is also much easier for boys to swim in the pool with swim diapers because there trunks cover them up. I saw MANY children swimming in the pools with swim diapers on under their trunks.
I am not stating that this is right for parents to bring their kids into the pool with swim diapers on but on our 1st cruise in 2001 any child could go into any part of the pools. Our 10 month old played on the mickey ears everyday in her swim diaper and I never had a second thought about it.