Both of these have the same response. The indoor activity is better for me. I sunburn easily. At the beach, I’d get sand and water in the book because I would be in the water and in the sand.
I would be more likely to set the book on fire, since I’d rather not be out reading on a lake. But, I think the risk of the book being permanently damaged is higher when it’s set on fire. At least if it’s dropped in a lake, I can dry it out and read it later.