It is my understanding that children in diapers are not permitted in any public pool by the Health regulations as a general rule. Therefore, it is not surprising that cruise ships would follow the same standards especially since they are so closely regulated by local health officials. As a Mom of 6, I fail to understand why you would want your little one in a pool that is so overly chlorinated in an effort by the cruise lines to limit the number of e-coli bacteria contaminating the water as well as other more dangerous bacteria that can come from fecal contamination of water sources.
FYI - there are others using the pool that although adults, may have some incontinence issues or, not have the best personal hygiene practices as well as youngsters who will also contaminate the pool water. As a longtime cruiser, I have personally seen cruise personnel having to close the pools due to youngsters and intolerable levels of fecal contamination that took place while kids were swimming/playing in the adult pools.
So my advice, enjoy your cruise and take your baby to enjoy the beach on all those beautiful beaches that you will be visiting while cruising.