That pitting behind the anode is the real story, once that zinc is half gone the current starts jumping to whatever bare steel is closest and it eats a crater fast. I pulled a 3 pound anode off a riser clamp last spring that was doing the same thing, looked fine from the boat side but the back was hollowed out like a sponge. First thing I'd do is grab a silver/silver chloride half cell or even a cheap multimeter and check the potential on that leg before you just bolt a new one on. If it's reading lower than -850 mV you got a bigger problem than one anode, could be a stray current from a nearby dock or a bonding issue. Also take a wire brush to the pit and hit it with a caliper, if it's more than 10 percent of wall thickness that leg needs an inspection, not just a new zinc. The deck hand probably saved you a diver bill down the road.