I live in Southern CA and have two Chesapeake Bay Retrievers. They do pretty well in warm weather here, but our "heat" is not nearly as oppressive as that in Florida. In the warmer months I do not exercise them in the middle of the day unless it involves swimming. I usually take them out in the morning and late afternoon. Mine are indoor dogs, but do enjoy sunbathing outside.
My husband runs with them and he wears a Camelbak (
http://www.camelbak.com/index.cfm)and carries a foldable water bowl in the Camelbak.
I know of a couple of great breeders in FL- Chesagrove Kennels (
http://www.chesagrove.com/) and Sailors Bay Kennels (
http://www.sailorsbaykennel.homestead.com/index.html). There is also a very active breed rescue in FL, Chesapeake Bay Retriever Relief and Rescue (
http://www.cbrrescue.org/). We got Indy from this organization, and he has proven to be a wonderful family pet.
Like you would with any prospective pet, be sure to research the breed before purchasing. Chessies are quite different from the other retrieving breeds. Chessies are hard, stubborn, protective, aggressive, one person/family dogs. But they are also silly, loving, loyal, kid-friendly dogs. I love the breed and would have no other.