Sea Dog Charters (mile marker 47.5 bayside, 305/743-8255, half-day charters from $60 per person) has four boats in its fleet, ranging 30–41 feet in length; their namesake boat is a 37-foot twin-diesel-powered sportfishing vessel that’s able to run deep-sea routes as far away as the Dry Tortugas and the Bahamas. It’s worth noting that Sea Dog is a “green” charter outfit that uses biofuels and recycled products.
The largest dive center in Marathon [1] is Tilden’s Scuba Center (mile marker 49.5 bayside, 305/743-7255, 8 a.m.–6 p.m. daily). The well-regarded crew here offers a large array of gear sales, rentals, and repairs, including hydrostatic testing, as well as air fills and PADI certification classes, reef charters, and the only SNUBA facility in the Keys.
Links:
[1] http://www.moon.com/destinations/florida/florida-keys/lower-keys/marathon