I noticed that my driver-side floor board gets wet. I talked to a friend and he said it could be an AC drainage leak because tractors tend to have a similar problem.

The floor only gets damp when the AC is on.

Any idea?

its just the drain from the a/c through the firewall. They get clogged or if it has been dissassembled for service it could be kinked. Locate it on the engine side of the firewall (on older troopers its by where the heater hoses go in) and clean it out with something flexible.