We've been through "several" thunderstorms in ours and to be truthfully honest I felt about as safe as any other shelter during such a storm. In our case all the boat cabin structure is built entirely out of wood, I can't say wood is actually be a safer choice but it may be less likely chosen as a lightning rod in any event.
Probably the most eventful thunderstorm experience came to us about four o'clock in the morning when we were nosed off on our favorite sandbar. I slightly awakened to thunder striking all around us and wind driven rain pounding on the roof and a cabin wall. Being on a river I knew it would rise rather quickly and float us free from our mooring but yet I was still unconcerned. I'd learned some time ago some folks like to play pranks with drunk sleeping sailors and I'd discreetly set my anchor out the night before to prevent such mischief and knew I wasn't really going anywhere.
Anyway about a half hour later I felt the boat beginning to shift slowly around as she was drifting free as the river raised. Even laying on my bunk I could feel the anchor dragging a bit at first before it grabbed tooth and the boat swung sidewise up against the sandbar and stopped, I just rolled over and went back to sleep again.
Along about dawn my drunken stooper was again interrupted by "the other half" who had awoken with the realization that we'd moved overnight and she wasn't comfortable with our boat parked lengthwise on the sandbar. She was determined I was going to address her plight without the comfort of some caffeine in my system to properly clarify and cleanse what remaining brain matter I still retained. And besides I knew we weren't in any danger of being stuck the water will rise for hours after a gulley-washer event we'd just experienced overnight.
So just to get her goat I put some water in the teapot to boil for coffee, made up my bedding and put it away and then set up the table. I then meandered out onto the front deck and surveyed the surroundings just as I do every morning when we're on the boat. I looked the anchor-line over checked it and them moved it to another mooring point and went in for coffee. The look on the Wife's face as she broke free of the sandbar and into the current a short time later was well "priceless". But then again I do like to fool with her it keeps her on her game.
But back around to your question.. I'm not much concerned about thunderstorms in general. But then again I also chose not to buy a nice stainless steel wheel for my boat I was eying simply because I didn't want to be the lighting rod standing in the cabin. But all in all I feel as safe in my boat as anywhere else I could be in a thunderstorm.
