The seabed had terrain as well. The Adriatic Sea could roughly be divided into three "steps." The north was rarely deeper than 100 meters, with the shallowest waters in the northwest, where Venice stood. That was why the Venetians had driven wooden piles into the lagoon and built the Venice of today.
The Central Adriatic Sea was roughly 300 meters deep, while the Southern Adriatic Sea reached 900 meters. This was measured by tying a weight to an especially long hemp rope and throwing it into the sea, then hauling it back once it touched the seabed. But the accuracy was highly questionable, since the water was moving, the ship was moving, and the rope could not remain perfectly vertical.
Palagruža Island lay at roughly 42 degrees north latitude and 16 degrees east longitude, approximately on the boundary between the Central Adriatic Sea and the Southern Adriatic Sea. This made the currents in those waters extremely complex.
The Dalmatian coast was lined with jagged islands. Those islands and Palagruža Island all sat on the same continental shelf, much as Corsica and Elba were part of the Apennine Peninsula despite being separated from it by the sea.
This region divided the currents while also leaving behind a "channel" that ran between Gargano, at the "spur" of Italy, and Palagruža Island. In winter, the Bora formed a west-to-east current in the waters north of Palagruža Island. South of the island, meanwhile, the currents of Ionia created an east-to-west flow. This current joined and intensified with the Bora Current flowing out from the "island bridge" near Barry, Italy. If one was unlucky enough to encounter a whirlpool, the water on the surface would be sucked down into the depths.
But if one knew how to use the currents—for instance, by spotting a landmark