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Maybe I have it backwards, but it has seemed from long experience with lightning rods at my home (27 years with no lightning strikes) that their function is to drain static charge off into the air and thereby to equalize potential differences that might otherwise attract a hit. From what I have been able to learn, the rods and connections are not intended to attract lightning, although they are sized to do so.
It would seem that an important element in obtaining such protection is a low resistance conductor that is continuous from masthead to seawater, just as it is important to have and maintain effective ground rods in the earth around structures that have lightning rods.
The principal problem with lightning protection in a vessel is the great difficulty of providing an effective connection to the water. Ground plates are deemed inadequate unless they have tremendous surface area, and external keels are often coated with material that is not helpful. Add to that the fact that the key hull to water interface seems to be at the waterline, and you get the idea that it isn't quite as simple as connecting a bond wire to a keelbolt.
Incidentally, it seems that lightning is a greater problem for boats in fresh water. That is probably because it is a less efficient electrolyte than seawater and therefore impedes the free flow of static charges.
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