I think I just found the answer to my own question.... posted at:
http://www.dcltribute.com/faq/toddlersinpools.htm
Can my toddler use the swimming pool?
Not unless your toddler is potty-trained. The United States Public Health Service (an agency affiliated with the Surgeon General's office and the Department of Health and Human Services) requires that children may only be allowed in the pools if they're potty-trained.
The pools onboard the Disney cruise ships are very small (this is true of the pools on any cruise ship). Although the pools are treated with chlorine, a single potty accident can cause oral/fecal cross-contamination to other passengers before the chlorine can do its job. Disney and a few other cruise lines had a well documented outbreak of Norwalk-type viruses last Fall. Although a specific contact point could not be identified, it is not unreasonable to take action against a potential cross-contamination site.
Enforcement of this regulation can be spotty. Some parents choose to ignore this regulation. Please don't be one of these parents. After all, would you allow your child to swim in the pool if they posted a sign that reads: "Warning: fecal contamination may be present."?