Not sure if OP, @xbainz013x , already made his decision but I used to live in Key West until recently. The planes that fly in are typically smaller aircraft as the runway is rather short. That said, the overhead bins can sometimes be a joke so definitely check out what aircraft you are flying in beforehand. The staff on the short flight from Miami to Key West are typically nice and accommodating, sometimes putting oversized items in the closet, but don't count on it. If you are flying into Miami and then driving through the Keys you may have more luck putting it in the overhead.
I just flew internationally with AA and for the first domestic leg with a smaller plane I was told by the gate agent that I would have to put my bass at the end of the load-in ramp for gate check. He gave me a red tag to put on my gig bag (Music Area Tang 30), which is not a hard case but is still fairly rigid with good padding. I got to the drop-off area right before the plane and stuck the tag in my pocket and walked on with the bag on my back. The flight attendant told me it probably wouldn't fit in the overhead and I told her I'll try anyway. The overhead compartments in the front of the plane were too short but towards the back, where my seat was, I got the bag to fit in with a little finagling.

So, in this case I was lucky to not have to gate check it.
On the second leg, which was the long-haul international leg, the AA staff welcomed the bass on board and took it to the front of the plane which I assume was to a coat closet or lay-flat seat in First Class

After the flight, they handed it to me as I disembarked.
Finally, the third leg was in Europe with British Airways and I HAD to gate check it. I had the gate agent contact a superior who came out and firmly denied my request to have the bass on board, stating BA changed their rules 6 months ago, yada yada. Additionally, the bass had to be in a hard case or else it would not go with me! Fortunately, the TANG30 was sturdy enough that he accepted it and so the bass went into the hold and I had to pick it up with the oversized luggage after getting through customs. That was a long flight but fortunately the bass was undamaged. Afterwards I checked BA's website and they have a clear-cut statement that musical instruments must have their own seat or else they will be in with the checked baggage.
SO, CHECK EACH AIRLINE'S POLICY REGARDING MUSICAL INSTRUMENTS AND SPECIFICALLY BASS GUITARS!!!
Hope this helps OP and others flying in the near future.