We went the week before halloween last year and had 4 days of wonderful weather, calm seas and really good prices. Not too many kids, plenty, but not overcrowded in the clubs. Spring break would probably have many kids and higher prices.
We don't worry about sailing during hurricane season-it could be a fun adventure to have a change in the island lineup.
We are going to try a 7 day next october, our kids have a week off during this time.