We did an Eastern last year September 22 - 29 and the weather was GREAT! Our previous cruises were in late November and early December and we didn't have good weather, so this was fantastic.
Quick rain showers for about 15 - 30 minutes when we were on St. Maarten and during our snorkel excursion on St. John.
It was hot and sunny, but I wouldn't say overly hot. The only time we really felt the heat was biking on CC mid-day.
As we were heading back to the ship from CC there was a big rainstorm, but it was the end of the time there, so didn't really have any impact on ruining our day there.
We definitely loved the weather on this trip more than the others, and I would do it again in a heartbeat!
Water was warm for the beaches and snorkeling, and it was just fantastic.