I am actually getting married on the Carnival Pride in 9 days. They actually marry you on board by an officiant. All we need is to take our marriage license, but we don't have to get married by a JP before the wedding. If you have any questions about a Carnival wedding I will do my best to answer them. Another great place to get information is theweddingexperience.com. That is who carnival contracts to handle the wedding stuff.
They also do vow renewals.
Hope this helps,