I am in Marathon now, been here two weeks. It’s my first trip, here is what I know so far, hope it helps
Lots of grunts, everywhere. Its the main fish you will catch.
You are lucky to get two good boating days a week in, the fan blows a lot. Today is 25kts.
Baitfish are fairly easy in a cast net. Look for balls of bait on your sounder in 5-8 feet.
Fish Washerwoman Shoals and/or Sombrero Beach Light first to get the feel of things.
There is a reef/hard bottom running for miles horizontal to the shore on the Atlantic side in 85-95 feet of water. Good fishing all along that line. You will see boats anchored about 3 or 4 miles offshore, they are likely sitting on that long reef.
Keep an eye on your sounder, lots of small patch reefs that can be fished.
Most everyone buys blocks of chum and floats them off the stern. Then you simply drop your baits in the chum line and wait.
Small hooks are your friend. The water is clear and the fish are line and tackle shy.
You can buy live shrimp, they are generally $4 a dozen and work well around the mangroves. Bait shops are aplenty for chum and shrimp. I havent figured out how to catch shrimp yet.
Island fish company has $3 stone crab claws at happy hour. The Poke bowls are awesome. Also sparkys has 30 cent wings and 30 cent shrimp at happy hour. Supper on the cheap and good eats.
There is a uninhabited key called Bamboo key on the gulf side of Marathon. We set up in 10 feet of water on the offshore side of Bamboo (about 1/4 mile offshore of the key) with chum bags and caught some decent fish one day.
With the size of your grady you should be able to fish the reef in 90 feet I told you about earlier, if it were me I would concentrate on that area, then branch out depending on how long you are staying.
Best of Luck