Not just that. Through radiometric dating, we have evidence of several billion years worth of nuclear decay. Nuclear decay is constantly releasing heat. It's a completely negligible amount of heat, but if you try cramming all 4.5 billion years worth of it into the single year-long event like creationists want to, it becomes a massive issue. Not to mention the other sources of heat like the water falling down as you brought up, the movement of continents, all the impact events, among other things. We're talking enough heat here to completely vaporize the oceans several hundred times over.

You think Noah had AC on the boat?

Edit: I forgot to mention the reason they need to shove all the heat into a 1-year event, rather than the 6,000-ish years they believe the Earth has been around for (not that it would be much better that way around). As the evidence of billions of years worth of nuclear decay is undeniable, Young Earth Creationists need some event to rapidly accelerate the decay by a factor of at least several hundred thousand times. And many of them say the flood caused it.