Don't be missed led, all diving in the keys can be great and can be poor. The same as anywhere. About the visibility, the Gulf Stream runs west to east along the keys. The clear tide water that comes from the south and Central Amers., goes by Key West first before it gets to Key Largo. Vis will change with the Gulf Stream and the tide. Vis may also change with the rain on the Keys. The reef is the same all the way along the keys, largo has good and bad areas, the same in Key West and all points in-between . You need to find a diving outfit that you like, the type of diving you like to do, I would recommend you try as many sites and different companies as you can, up and down the Keys. I recommend you stay with a smaller boat (not to small) instead of the bigger cattle boats. The smaller boats can tailor to your style of diving. I deal with conditions on both ends of the Keys, good and poor. I have been on the Bibb& Duane in great conditions and some real poor vis. and big current also. The same goes for the Eagle, Tbolt, Busch, and Cayman. Some years ago before the mooring balls were installed on the wrecks, we had to bounce the wrecks to get moored. I came off the boat to hook into the Cayman Salvor in Key West area. We could see the wreck from the surface in 90+ feet of water, but when I hit at water from the surface I could see the complete wreck, all 185'. Thats 250+' of vis. other times I could not see the wreck 15' away. The point is no one part of the Keys is better then the other, There are some different things about all the areas. Have fun and go look & try as much a you can.
RMC
Big Pine Key, FL.