With descry and the more common decry ("to express strong disapproval of"), we have a case of linguistic double-dipping. That is, English borrowed from the same French root twice. Both words ultimately come from the Old French verb decrier, meaning "to proclaim" or "to decry." English speakers borrowed the term as descry in the 14th century and used it to mean "to proclaim" or "to spy out from a distance" (as a watchman might), and eventually simply "to catch sight of" or "discover." Meanwhile, in French, decrier itself developed into the modern French décrier ("to disparage, to decry"). English speakers borrowed this word as decry in the 17th century. Be careful not to confuse descry and decry. They may be close relatives, but in modern English they have distinct meanings.
Verb we couldn't descry the reasons for his sudden departure could just descry the ship coming over the horizon
Word History
Etymology
Verb
in sense 1 Middle English descrien, descriven, descreven "to catch sight of, discover, discern," apparently developed from descriven (rarely descrien) "to describe, recount, characterize," borrowed from Anglo-French descrire, descriver, descrier "to describe, give an account of," going back to Latin dēscrībere "to represent by drawing, describe"; in sense 2 Middle English descrien "to announce, make known, reveal, betray," perhaps borrowed from Middle French descrier "to cry, make known," from des-de- + crier "to cry entry 1"
Note: The history of both semantic branches of this verb is difficult to characterize, given that the meanings of individual occurrences in Middle English are often not obvious and may overlap. Sense 2 is generally traced to Middle French, though evidence for the word in French texts is exiguous, and instances of this meaning contemporary with Middle English uses are lacking (see Dictionnaire du Moyen Français, on line). A putative descrier "to announce, proclaim" in the Grandes Chroniques de France (late 14th century) should be read descirer (modern déchirer) "to tear up" (referring to letters brought by Philip VI before the Parlement of Paris). There appears to be no connection of the Middle English word and Middle French descrier "to devalue, discredit," not attested before the 15th century (see decry).