Zinc anodes depend upon a medium such as water to carry the electrical charge, most commonly in the areas of turbulence around the rudder and shaft. The charged water treats the zinc and the steel as one homogenous substance resulting in the zinc's erosion. It's not the zinc's attachment to the steel that does the job as much as it is it's proximity to the flowing water which imparts a difference in potential or current. That force is absent inside on the keep bolts. The effects are similar but the processes of corrosion are different. Were they similar, your car would come equipped with zinc's and my Green Goblin '97 Ford pick-'em up wouldn't look like a gunnery target.
You can use the product bubb2 lists above or you can just smear some grease on them. If you're picky about your grease, you can use silicone grease, allowing you to eat any french fries you drop on it.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.