Marathon.
Last trip we stayed at the Seahorse cottages. They were pretty nice, but the real plus was fishing out the back door. Lots of current in that canal cause it's a block off of Vaca Cut. You can catch supper every evening off your back porch.
Fishing off the bridges is great. Fish in the shade breaks and around the pilings, always plenty of food to catch. Blue line grunts are as good as snapper to eat.
Get familiar with a knocker rig (egg sinker and a hook) keep it simple. Fresh bait. Always carry a few sabikis. Chum is always suggested. Frozen blocks of chum are everywhere. If you are fishing, you should be chumming. You need a chum bag(net) as well.
Small pinfish are a premier bait for almost everything. Squid and cutbait is fine for grunts.
I don't know nuthin bout snorkeling beyond this:
"If you look like a twinky, and swim around in my kitchen, I'm gonna at least taste ya."
I don't swim where sharks are plentiful, and they are shurnuff plentiful in the keys.
I like the 7 mile grill, they are always real nice and the food ain't bad, it's also the most affordable meal down there. They also have the best key lime pie I have eaten in the keys.
Stay TF away from the Cracked Conch Cafe, the worst( and most expensive) meal I've eaten in the keys. Any place down there will cook your catch for you.
The Marathon Lady is a good headboat for a guaranteed mess of fish. I'd definitely take my own bait and gear though. If you go take some ballyhoo for cutbait. It'll target the nicer snapper.