Having lived aboard a forty foot sailboat for years and knowing a large number of liveaboards, I can think of only two instances where someone on a boat was serious injured by a lightning hit. Generally when a boat is struck, the lightning pretty much goes everywhere and destroys a lot of electrical gear, but aside from burns, it doesn't electrocute anyone because they are not grounded (or because it chooses not to, who knows).
A sailboat is like a metal pole forty or fifty feet high (even if it's a wooden boat and a wooden spar, the mast has electrical lines that run to the top of the mast) saying 'hit me' in the middle of the ocean, so strikes happen with some degree of frequency.
When your 200 miles from land or even anchored in Biscayne bay, you're stuck in the storm when it comes up, you can't get out of the pool.
On the question of being on the surface, I think the likelihood of being struck would be extremely low because your profile sticking above the water in so low. Lightning works in a counter intuitive way. It doesn't 'strike down' ionised particles discharge up to the cloud and then the visible lightning follows the 'path' back 'down' to the grounded object. This accumulation of particles tends to gather around (or ionise around) objects with some height and often with irregular features, say an antenna or electric pole. I don't think, and I may be wrong on this, lightning just strikes the ground in an open field, at least not very often.
Stan
