If it is big fish you want, you will want to be trolling. When you are down there the most active big fish will likely be King Mackerel. These fish are a larger, nastier version of a Muskie, and have teeth that are sharper than those of most sharks. Do NOT let any part of your body near their mouth while they are still alive! They are also very aggressive, often jumping out of the water to hit the bait on the way back down.
The most popular fish this time of year is the Sailfish. I had the wonderful experience last year of being part of a group of 3 that boated 4 sails, from 5 1/2 feet to 8 feet in length. These fish fight just the way you have seen them on TV shows, tail walking, diving, running, etc. Again, you want to keep clear of the mouth, as that bill of theirs will put a nasty gash into anything it can make contact with. Sailfish are not kept, so be sure to have a camera handy. (the one thing we DID forget on that trip). If you are interested, the boat we went out on was the Jolly Roger 2, out of Garrison Bight Marina in Key West.
The other fish that you may run into are baracuda, tuna, bonita, and if the water is a bit warm, dolphin (mahi-mahi, not the mammal).
If you are on a budget and are interested in some decent fishing from a Party Boat, the Greyhound is the way to go. A 6 hour trip for about $35, and you will be almost guarenteed to catch fish. You can also get some large fish on this boat, but you will be fishing bottom with cut bait, so you are targeting Grouper, Snapper, and other fish that like their food not to move too much. Trolling trips will probably run from $400 – $1200 for a trip, depending on the size of the boat, if it is a full or half day, and if it is trolling or anchored. Many boats are willing to set up a split charter, so 4 – 6 people will end up sharing a boat, and will also be sharing the time in the chair, and sharing the catch.
I am down there every year for a week. This year it starts Jan 6. If you (or anyone else) are going to be there when I am, give me a PM.
And don’t be afraid to walk around the marina, asking questions. Often when a boat doesn’t have anyone for a trip the next day, they are willing to negociate on price a bit. They would rather take someone out for a little longer and/or for a little less, than be sitting idle. If you want to see what they are catching, the morning trips come in from 12-1, the afternoon/all day trips from 4-5.