Nancy is the sun protection expert around here but I'll take a shot.
I am fair skinned and my experience is that the sun is pretty strong down there in October. I would go for at least SPF 30 or higher.

You have to keep an eye on the clock as well - it's easy to lose track of the time as you sit in a beach chair in the water and have people bring you drinks. You need to reapply as well.
Just a word of warning - I found out from the very smart people on my last cruise that suntan lotion has an expiration date! (who knew?) And from what they tell me, it will give you a lower level of protection if it is expired. One person reported getting a fairly bad sunburn after using SPF 30, checked the bottle and found that it was expired. So it you buy an suntan lotion now, make sure the expiration date is past the end of this year.