The bigger the sailboat, the more likely it is that it will have a grounding rod through the mast and keel, then out the bottom. Or at least thats what I've been told by they sailors who travel the Great Lakes all summer.
In the event of a strike, it will direct it through to the water.
There still can, and likely will be damage, but it will not be traveling through your body to get to the water.
But actually, according to my physics teacher at the UW, the direct ground from water to top of mast will act to dissipate the electrical charge which attracts the negative charge of the electrons in the sky. So if there is no attracting charge on the boat, the likelihood of a direct strike is greatly diminished.
This is the same system used on barns and large buildings for years and years. Those cute little lightning rods on the roof dissipates the charge so the lightning is not attracted to the building.
According to NOAA, 14 people were killed by lightning while boating between 2006 - 2013, so less than 2 per year.
Just as many people were killed while camping, or playing soccer.
Another 30 people were killed while fishing, including from bridges, piers, or shore.
Still less than 6 per year for boating and fishing combined.