Capt Mel is spot on with his info. If you want to flyfish with a guide make sure he is a flyfisherman. Another good question to ask is this (and word it just so), "What is the best way to fish for fish X?" If he says anything about chumming or bait or casting jigs or plugs then say thanks and call somebody else. A lot of sw guides have gotten into the flyfishing thing and they don't flyfish themselves so they don't know how the boat needs to be positioned and where you need the wind and a myriad of other things. These guys are great anglers for the most part but most can't help the regular fly angler who only has one chance to try for a fish they may have never seen before and want to catch it with a flyrod. Good luck on your trip. I don't know much about Key West specifically (I fish Islamorada area whe I go down there) but if you can find a light that hangs out over the water, I am pretty sure you will find tarpon under it at night, and if you throw a small white streamer of some kind at it, you will have a pretty good chance at hooking it.