Reference Chapman's Seamanship (I didn't write it...) for information on lightning and the cone of protection. The idea is related what is called the Farraday Cage - surrounding yourself with conducting stuff to attract and direct the hit away from you. What apparently has been realized with boats is that doing this also seems to help bleed off potential, making the boat less attractive to hits in the first place (same thing with other tall structures properly grounded). Boats still get hit, but I'll go out on a limb and say it must be effective to some degree - there are acres of keelboats around, and rarely do I hear of one being hit. I've heard of a powerboat getting hit while underway - strike hit the ungrounded VHF antenna and blew apart the console, nobody killed but it ruined the day.