I'm far from an expert on lightening, but do know a fair amount about electricity in general...
I think the big difference between a boat and plane is that the boat is grounded where the plane is not. Lightening is electrical charge seeking a path from the charge source (the sky) to ground. As psneeld said, it will follow the paths of least resistance. This path can include your boat, because you boat has a good ground connection via the water. When that happens, there is a massive amount of energy passing through your boat to the water, via every path it can find.
I would venture that planes aren't "struck" by lightening the way a boat can be, all because the plane isn't grounded. There is no preferred electrical path through the plane the way there is through a boat. So I would think that planes are more in proximity of lightening strikes, and not so much directly in the circuit of a strike.
Now of course this is all statistical, with no hard rules. In the end, there are infinite electrical paths from the charge in the sky to the surface of the ground. What paths the spark ultimately takes depends on the make up of the atmosphere, shape and contour of the ground surface, and material makeup of the ground surface, etc.