What should I do immediately after a clean water flood to minimize damage?
DIY: Ventilate, ventilate, ventilate. You want to RUSH to Home Depot and get some of those giant fans ... plus a couple of dehumidifiers. If you have air conditioning, crank it as low as it will go for a few days. 

Personally, I would call in a company like ServPro that does emergency water remediation to evaluate and help fix the problems.