hmmm interesting question. I'm not sure there is a pat answer though as this really can be different for everyone. But I can offer this- We moved from Chicago to Florida (no longer live in Florida) and noticed that everyone that came to visit would insist that would not burn despite our warnings on using lotion. Most everyone would be lobster red the next day. So, based on our experience, anyone that goes from a winter climate to a tropical one, needs to use at least an spf 8 and then only stay out in the sun no more than a couple of hours. This will start a based tan and than you can drop the spf down the next tanning session. If you are going on an excursion where you are going to be on the beach or in the sun most of the time.. then beef it up to a 30 (I agree, anything higher than 30 is a waste) Make sure that you cover your face and ears if you are not going to be taking clothes off. (Say going to the Mayan ruins) Many people think that since they are not going to be sunbathing or beaching it, they don't need it.. only to have the redest nose and ears on the ship the next day)
I have had good luck going without anything and doing a quick 15 mins on each side tan session and then going inside, letting the skin react (about an hour or two) and then doing a longer session with the 4 or 8...
But, like I said it really varies person to person.. If you burn easily, I would trust anything less the a 15. I have seen people get a burn through 15. The tropical sun can be brutal.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013