Since the Bahamas lie in the Hurricane Belt, low season starts around Labor Day. That is when many hotels plan their "Fall renovations", etc. You can still have beautiful warm and sunny days and nights.
i live in south florida on the east coast. bahamas is just off the coast of ft. lauderdale so they have the same weather we do. the summer means rain. the summer means hurricanes. that is why there are tons of real cheap cruises to the bahamas in the summer (summer is june - sept)