since you are leaving in 5 days, do you have a bit of a tan? If you do then a factor 30 would be just fine- leave the 15 at home. If however you have no base and are pale, then definatley go for the 45. Also don't forget sunscreen for your lips.
Buy waterproof sports-type sunblock and don't forget to put it on the tops and Bottoms of your feet. If you are swimming the glare off the water will burn the bottoms of your feet, that's a big owwiee,if you burn the bottoms of your feet.
I always start by going to a tanning spa about a month before the cruise, just to get a base started....then I start with a 15 once down there. This helps so much, haven't gotten a sun burn when doing this....just be smart and keep yourself protected from the sun.
My very first cruise I got major burns the very first day. Be very careful the sun is particulary strong on a ship and the slight wind sometimes is deceiving so you dont always feel the intensity of the sun. Last cruise we pretanned first and it did help.
All dermotologists say go minimum 30.
I have a friend who refuses to sit in the sun at all for fear of skin cancer.
Plus be sure to wear a hat and take aloe vera in case you do burn.