Boats get struck by lightning all the time. The amount of damage depends on a variety of factors, and there are no guarantees.
As a general rule of thumb, the best advice is to seek shelter off the boat, but of course that is not always practical. Failing that the general advice is to stay away from metal objects. That only goes so far.
There are all kinds of gimmicks out there such as these little brush things for the mast head, but the current thinking is that none of this stuff does much good.
The way the boat is built can help some. If there is a direct path for the lightning to pass through the boat to the water, the damage is less likely to be structural than if there is not and the path of the lightning passes through the fiberglass or a bonding system.
So for example my boat was hit while out of the water. Our best guess is that the lightning hit the masthead, came down the mast itself, through the aluminum mast step, through the stainless steel beam that the mast step is bolted to below the cabin sole, through the stainless steel keel bolts that pass through the stainless steel beam and out through the lead keel. While the electronics were fried, there was no other damage.
By the same token, when I worked in boat yards in Florida, the yard did an emergency haul-out of a boat that had been hit by lightning. The lightning had apparently passed through the shrouds and chainplates, into the bonding system and in part passed through the thru-hulls and in part passed through the hull. The copper bonding straps were melted and had gotten hot enough to set the bulkhead and fiberglass on fire. The thru-hull had 'de-zinced' and could be crumpled in your hands (it held until the boat was hauled and was discovered during the survey. A portion of hull was riddled with small almost microscopic pin holes where the electricity passed through. Since then I have heard of other cases of 'worm holes' in glass boats struck by lightning.
Recently an article in Sail Magazine discussing lightning and while a little superficial, it did provide a good general description.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay