If the autorotation is successful, the sea is calm and all the floats inflate, the aircraft will remain upright for a while.
Sinking is generally caused by the sea state rolling the aircraft over, or a float bag slowly deflating and causing a rollover.
One of the large Gulf of Mexico operators who have recently lost some aircraft, all which have had successful autorotations, is looking into ways of increasing the longevity of the aircraft floating so that they increase the chances of aircraft recovery.
In the cases which occurred recently, the aircraft was lost, and the operator believes that improved floats or supplementary floats could prevent hull losses.
Brother