In respect to Jellyfish, in my experience ( pushing 40 years in the Caribbean now ) the summer months in the week before the full moon are the worst. Actual location doesn't seem to have much to do with it, it's just that some places are more visited than others or the people visiting them choose to report more often. So if jellyfish are a big deal for your crew, schedule your rip near the new moon or at least after the Full Moon as the turtles would have had a chance to eat the jellies as it is a favourite food for them.