Key Largo's dive season runs year-round, but conditions are best from November through April: water clarity is at its peak, there are no thunderstorms, and temperatures are pleasant rather than oppressive. This is also peak tourist season, so accommodation costs more.
Summer (June to September) brings heat, afternoon thunderstorms, and humidity. Visibility can be reduced by plankton blooms in the warmer water. The upside: fewer visitors and lower prices. Hurricane season runs June through November; September and October are the highest-risk months, and it is worth watching forecasts if you are visiting then.
The lobster mini-season in late July (the last Wednesday and Thursday of the month) is the busiest two days of the year: thousands of recreational divers converge for the pre-season harvest. Regular lobster season runs from August through March. Tarpon season peaks from April to June, drawing fly fishermen from across the country.