We have chartered in the BVI in September, anchored out the whole time, and it was unbelievably hot. I mean, so hot that we couldn't sleep. I'm not kidding! Hot, sticky, sweaty. Sometimes it would rain at night and we'd have to close the hatches. Then it was beyond unbearable.
We've chartere in October and it was the same way... the charters in November, December, February and April were much more comfortable.
We had a fantastic time on the September/October charters, appreciated the fact it was much cheaper and the anchorages
were all available with no crowds, and we loved every minute of our vacations, so go anyway.
But just be aware if you don't have a/c you'll roast, even stark naked in the cockpit