Mike, there's never a guarantee on the weather, but Aug-Nov are generally the hurricane season, with a greater chance of big storms in the Caribbean. However, big storms can occur anytime. Early in the year (Jan-early Mar), or in May before schools let out, prices often are somewhat lower than during peak times (spring break - which is usually mid-March through April - and summer).
Again, no guarantees. Some of my friends encountered rough seas in March, and I did in January, though the weather was not stormy. We made the best of it and enjoyed it anyhow!