Uh, no autopilot has the ability to land a plane (intact) on water.
Only if the engines are running, driving the generators and hydraulic pumps. An autopilot set for cruise will maintain altitude as long as possible (read: Until all fuel is exhausted), and then run for some time on battery power. If the Ram Air Turbine is not deployed quickly (and the autopilot by itself is incapable of doing this), the hydraulics will soon fail to work and whatever commands the autopilot sends will be ignored by the control system. Bottom line: The plane will descend uncontrollably. Not land, but crash.