i know various airlines differ but i'm a purser on an a320 and barring obvious conditions such as fire or blockage all exits are useable in a ditching. we used to operate 737-300/700's and the brief was the primary exits in a ditching are the overwings, and if possible the foward doors but not the rear doors i believe it was something to do with the boeing being very tail heavy.
my congratulations to the whole crew though! awsome job guys