What is the difference between 'decry' and 'descry' ?

They derive from the same source, but their uses now are quite separate.The common source is the old French 'DESCRIER' meaning  ' to cry down'.

DECRY

To decry, taken from modern French, means to belittle or disparage openly, to censure or denounce.

• He decries the lack of rehabilitation measures for the earthquake victims.

DESCRY

To descry comes from a Middle English form of the Old French, and has moved from the original sense.It means 'to discern something difficult to perceive, to catch sight of' or 'to discover by careful observation and investigation' :

• They descried a ship through the mists.