How would you implement this?

I'm going to need to implement aiPlayer walking on the deck of a ferry, where the ferry is animated as if it's floating on water (i.e. it's rocking back and forth and bobbing up and down, including both translation and rotation in the animation). How would you implement this, so that the player's feet stays reasonably close to the surface of the animated deck and doesn't fall through when it moves up, and still can walk?