Got another question for some of you "old timers"

I have spoken to other boater's that keep all their gear in the boat all summer, Can the summer heat cause any problems to your rods, reels, fishing line, plastic baits, ect?. If kept in the boat covered up when not on the water?
When we had all the heat last week, I took off my cover and everything was almost too hot to touch. (a bit like that U.S. Army tent you got to sleep in, It had no place to vent the heat, The heat would cook you if you didn't roll out at sun rise)
I know on the side of the box your fishing line comes in, it reads "keep out of the sun", but it doesn't say anything about the heat of the sun. I also worry about the effects the heat will cause the oil to dry out in your fishing reels.
Anyone have any thoughts on storing your gear "in" or "out" of your boat during the summer months.

Thanks for the insight