Text in force at CS23-6: from CS23-5, effective 15 Aug 2017.
Aeroplanes intended for operations on water must:
(a) provide buoyancy of 80 % in excess of the buoyancy required to support the maximum weight of the aeroplane in fresh water; and
(b) have sufficient margin so that the aeroplane will stay afloat at rest in calm water without capsizing in case of a likely float or hull flooding.
ED Decision 2017/013/R · Source