Its not directly the movement of water, but if a organic being (read: carbon based life form) ever dies, it will deposit carbon where it rots. So if any seaweed or fish or unfortunate land creature, etc. dies in a flood, then carbon dating becomes problematic as bits of flesh can be hidden in places that we would not expect, and leave no evidence after decaying. Same goes for lava flows (which screw up geological dating even more due to other factors) and other acts of nature.