2. Don't panic in water (this is the hardest part for me)
3. Snorkel with someone you know (I even broke this rule).
4. Know how to use your snorkeling gear (esp. how to spit out water if water gets into your mask)
I snorkeled in Hawaii, Mexico, and outside Key West - Dry Tortugas NP without knowing how to swim. I finally learned swimming, figuring it would be more fun.
I am under the impression Key West itself does not have good places (if any) for snorkeling, neither does it have decent beaches. It is on the other Keys or on Dry Tortugas NP.
November water temp should be warm enough - the first time I went to Key West was around Thanksgiving. Water was fine for snorkeling in Dry Tortugas. Wear sun screen.