Why are the 737's rear doors unusable in a water landing? The 737's rear exit doors cannot be used to evacuate the aircraft in the event of a water landing, as shown, for example, in this safety card:



(Image from flight-report, via Jordy here at AvSE.)

In contrast, the rear doors on (for instance) the A320 series can be used for a water evacuation:



(Image by Czechnology here at AvSE.)

Why can't the 737's rear doors be used during a water landing?
The bottom of the door opening sits too close to, or below, the water line when the airplane is floating.