reeltherapy:
Swordfish can be caught down there. Like LHPfish mentioned, forum member/moderator "Gottago" fishes for them regularly, uses a mix of artificials even. His name is Rob Harris and he lives in Cudjoe Key. (I think he's got a Tackle shop going as well.) So he is a good number of miles SW of Marathon.
I have spoken with him before and there are a few select spots in that area that are better at giving up swordfish. Generally called Wood's wall in that area. I hear that other people are catching them down there as well, just that they are not advertising it much.
But even if you try it off of Marathon you may probably still want to drift along that same roll-off line. The only drawback is that we have it pretty good off of Miami-Dade and Broward: we have only 15 miles of offshore travel plus your port of call angle. Off of Marathon to Cudjoe it is about 25 miles due south to get to the zone plus your port of call angle.
But if you are going out to 500-800 feet of water to get dolphin (not sure how it goes in winter time), you will be a good ways out anyway. You may as well just make it a long day and stick it out till sundown and beyond and try doing some swordfish drifts, or make an early day return trip to spot stuff on the way in. It is a bit of a drag to set out simply to go swordfishing without making some other type of fishing drift while you are out there. Figure 60-75 miles of running for a round trip and it may make a lot of sense to leverage your effort.
Good luck. You may want to pick up a Home Port Chart of the area if you do not already have one. A marine shop in Marathon carries them (West Marine I believe). That small shop in Big Pine next to the Ace Hardware store carries them as well.
I haven't heard too much about big fish being caught in that zone. But you can set the record straight in May.
(Of course, your alternative launching points can change everything.)