When I say summertime fishing in Key West what I am more referring to is July, August and September. In terms of fishing the change from spring to summer takes place more somewhere in July.
Typically in July the water will start getting hot and you will algae blooms which tend to turn the water on the reef green. The warm water is not as good fishing for some species of fish like grouper, yellow jacks, and amber jacks.
A lot of fish will move deeper this time of year, shallow patches that were on fire a couple months earlier will turn into ghost towns.
The snapper will be still around on the reef and in good numbers, as it gets later in the season the gray snapper on the reef tend to get smaller, as the bigger ones complete their spawn and head back to the gulf. These fish can pretty smart and sometimes it will take some live bait to catch both the larger yellow tails and larger grey snappers.
Bottom fishing in the deep water will be very productive. Also the gulf can be good this time of year but you may have to run farther then when the water was cooler.
August marks the mini sport season for lobster the beginning of the regular lobster season, so that will be red hot for the first month or so.
The seas tend to be calm during this time period so although the fishing might be a big slower, it will be very easy to run further distances so sometimes that will make up for it. This can allow for longer trips such as the Air force relay towers in the gulf of Mexico.