Ed,
Of course the seas can drop at any time, but the most reliable time for reasonable seas is the last two weeks of July and first couple of weeks of August...depending of course on Hurricane situation. Late July is the prime time, in my experience. As the sun angle begins to change and the days shorten somewhat, I've always noted less fish and that usually begins to be a factor in late August. You can still find scattered fish through Sept. and later, depending on weather, but the numbers begin to decline in late August as the sun angle changes.