From Odessa: Genius and Death in a City of Dreams, by Charles King (W. W. Norton, 2011), pp. 33-34:
Pirates from north of the Black Sea frequently targeted Ottoman ships, even hitting the Ottoman heartland in Anatolia and occasionally menacing Constantinople. These raiders grew up out of the frontier society that defined the coastal borderlands of the empire—a mixture of former Polish-Lithuanian or Muscovite peasants, local Muslims, and nomadic herders, some of whom coalesced into distinct communities given the catchall label “Cossacks.” Cossack groups emerged in the mid-sixteenth century as a key power at the intersection of Polish-Lithuanian and Ottoman authority, offering their services as freebooters—the word “Cossack” probably derives from kazak, a Turkic word for “free man”—to whichever sovereign could pay the highest fee. Although a substantial livelihood came from raiding and piracy, Cossacks were a true multipurpose frontier people, farming, herding, and fishing in the grassy lowlands and estuaries of the Dnieper and other rivers.
The French artillery engineer Guillaume de Beauplan, who witnessed Cossack raids in the seventeenth century, left a graphic description of the Cossacks and their waterborne lives, painting them not as the legendary cavalrymen they would eventually become, but rather as able and daring seamen, commanding small rivercraft that could be reoutfitted for voyages across the sea. As he wrote in his Description of Ukraine:
Their number now approaches some 120,000 men, all trained for war, and ready to answer in less than a week the slightest command to serve the [Polish] king. It is these people who often, [indeed] almost every year, go raiding on the Black Sea, to the great detriment of the Turks. Many times they have plundered Crimea, which belongs to Tatary, ravaged Anatolia, sacked Trebizond, and even ventured as far as the mouth of the Black Sea [Bosphorus], three leagues from Constantinople, where they have laid waste to everything with fire and sword, returning home with much booty and a number of slaves, usually young children, whom they keep for their own service or give as gifts to the lords of their homeland.
As the Cossack raids illustrated, in the seventeenth century at least, the Ottomans exercised little direct control north of the Black Sea, except during seasons of war when troops might descend on local villages to burn crops or requisition livestock.