Right side up people (those who live in the northern hemisphere) believe that they can never see The Southern Cross in the sky. Similarly, upside down people (such as Aussies and Kiwis) believe that they can never see The Big Dipper. Wrong on both counts. People in the zone 25N to 25S can see both constellations at certain times of the year. Marathon is just at that 25 N boundary, and right now we are close to the winter solstice.
I got interested in this a few years back. We were sailing one night in The Hawk Channel, heading for Marathon. I happened to glance over my left shoulder, and there were four bright stars obviously suggestive of a cross shape. "Must be the Southern Cross," I thought despite the fact that I had never seen The Southern Cross before. My second thought was, "Nah, that can't be. We can't see it from here."
Well, I checked out that assertion using Stellarium, the free planetarium program for your PC or MAC. The screen shot below shows how the sky will look at 05:32 tomorrow morning from Key West. The Southern Cross sits just above the letter S. All four stars are above the horizon only 30 minutes.
But seeing it won't be easy. Inside Boot Key Harbor, the view of the southern horizon is blocked by buildings and mangroves. You need to be anchored out in the Hawk Channel. Even then, the southern horizon is hidden behind the clouds that hang over The Gulf Stream most nights, so you'll have to find an exceptional day.
But the more the obstacles, the sweeter the victory if you do see it. Good Luck