Don’t you have better snorkeling destinations on your cruise? The reef here is a few miles offshore and all you are likely to see is some fish from the beach, assuming good conditions. None of the beaches here are very nice.
Personally I would spend my time here seeing our historic district, sampling local, fresh seafood and taking in the unique ambiance of Old Town Key West, but it’s up to you. Fort Zachary posts water visibility conditions at the park entrance. If it says fair, or poor, don’t bother.