Was driving through puddles yesterday and got in shit from my dad telling me it could crack the headers on my truck. Just wondering if he is full of it or not.
Short answer: Not likely
Long answer: Theoretically, any hot metal surface could crack when it undergoes thermal shock. In an old truck, the engine block and head are likely cast iron, but I'm not quite sure about the headers. They could be formed steel or cast iron too, but that would vary more. That said, it would need to be a deep puddle to really get enough water to the right places (ex. the headers) to generate enough thermal stress to cause a crack. Overall, unlikely, but not impossible.
It's also possible that you can flood the engine if the air intake is low enough. This is more of a concern when driving through flooded streets, not puddles. It's also why you'll occasionally see off-roading Jeeps with black plastic ducts leading over the roofline. They're called snorkels and they allow the vehicle to traverse deep water, like streams, without flooding the intake manifold and the engine.
Another thing of note is that you don't know what's in the puddle. You could easily hit an obscured obstruction, like a pothole, at speed and damage chassis components. Perhaps there's a deep mud patch that sucks your tire right in. Suddenly, you're stuck. Damaged, no, annoyed, yes.
My last note is that hitting puddles at speed can be hazardous (thus causing damage from collisions). Water is significantly denser and more viscous than air, so driving through it creates immense drag. If you hit it with both wheels, you could decelerate pretty quickly (so buckle up). If you hit it with only the left or right set of wheels, you'll get pulled toward that side. Either scenario is risky.
That all said, I do enjoy creating large splashes by driving through puddles.
I think the bigger concern is getting water into your intake. This is even more of a concern if you are running a "cold air intake" that is not protected by an airbox. There are other concerns around breather tubes for diffs and electronics, among other things. As far as the header goes, I'm inclined to believe this isn't an issue. I think about dirt bikes with giant exposed exhaust systems that get wet without damage. Who knows though, sometimes the old man knows his shit.