Does CFIT apply in case of a plane (not an seaplane) failing a "landing" emergency in the open sea?
Boeing defines it in the following way:
Controlled flight into terrain (CFIT) describes an accident in which a flight crew unintentionally flies an airplane into the ground, a mountain, water or an obstacle.
Since it's not unintentional, it's better described as forced landing or in your case, ditching.
Controlled flight into terrain means that the aircraft is under positive control, that is it's within its flight envelope and is being actively managed, however it's course brings it unintentionally into contact with terrain. A aircraft flying into the side of a mountain is controlled flight into terrain. An aircraft ditching, successful or not, is an intentional action so doesn't qualify.