Hey, check out a cut on the north end of Marathon called "Vaca Cut". US 1 goes right over it. On the Gulf side of the bridge there is a channel cut through the mangroves and rock going to a private marina that's owned by a resort hotel. There are ledges cut out in the coral from the creation of the channel the goes to the private marina. This spot hold a good number of fish, is right by shore, and should be fun for you and your family. Try to cast net live pin fish, whitebait, or other bait fish on a grass flat and keep them in a live well or well circulated bait bucket. Using canned cat food is a good way to attract bait fish and it's cheap. I like to use a light-medium spinning reel such as a Penn 4400 or 4500 with 8-12lb line. In the clear water fluorocarbon leaders are a big help for pickier things like Snapper. 25-30lb leaders should suffice. For hooks with live bait I use a size 2 short shank saltwater hook. This is a fine size and is especially good for not spooking fish with a big hook. Use 2-3ft of fluorocarbon and tie this to your hook. Then take a barrel swivel and tie to the other end of your fluoro. That's your leader. Slide a 1/4 ounce egg sinker onto your fishing line and tie your leader to your line. This will allow your bait to get down to the fish in the current and keep your sinker far enough away from the bait that it won't scare fish. Anchor your boat near the intersection of the private channel and Vaca Cut but not on the intersection. Hook your bait either through the lips, eyes, or the rear quarter and toss out where Vaca Cut and the private channel meet. Also try tossing the live bait right along the coral walls at the entrance to the channel. I've laid waste to a good number of fish in this spot, like tarpon, a variety of snapper, grouper, small amberjack, sharks, barracuda, etc. Live baiting is in my opinion most effective but jigs also work. For those use a similar leader minus the egg sinker and with a 1/4 ounce jig head and some type of soft bodied jig, like a berkely gulp. There are a lot of bait and tackle shops and a World Wide Sportsman on Islamorada where you can ask questions and get tackle. Don't be afraid to ask, the people are pretty helpful. If you have any other questions please ask, and if my instructions are not very clear I can attach pic's of my rigs. Also try the other side of the bridge in Vaca Cut (Atlantic Side) at night with heavier tackle and decent size(4-6inch) live bait on the bottom for bigger fish. Anchor in the channel, with lights on of course, about 100-150yds away from the bridge.
By the way, my wife and I are going there for our one year anniversary in May and you can bet we'll be hitting that spot. The key's are a great place, just watch out for Key West. Let me know how you do. Good luck.