The island group is part of the Cretan Islands archipelagoes. They are on an off-ridge of the submarine range to which Crete belongs, the
Hellenic arc, the curved southern border of the
Aegean Sea from the Peloponnesos to Rhodes. To the south, the
Hellenic Trench, as much as deep, borders the Hellenic arc, while to the north the
Sea of Crete, a deep adjunct of the Aegean Sea, descends as deep as . In contrast to these deep water environments, the coastal shelf of Crete is shallow and dotted with reefs, making it dangerous for ships. The Dionysades lie in shallow water. To the north of them the coast drops off.
Hydrology A British admiralty charts of the late 19th century, plotting the
soundings assiduously and with some danger taken by
HMS Spitfire (a paddle gunboat) in 1852, published a visual account of the coastal shelf around Crete (see maps below). The shelf roughly follows the coastline, but further out. A dotted line marks the virtual outer border, taken to be the line, developed by intuitively curve-fitting on the outermost soundings. Beyond there the bottom drops into the deeps and soundings were not possible then. Because of subsequent revisions the chart should not be used for any current navigation, but it does give an approximate view of the shallows. Soundings immediately off the south of Yanisada are around , dropping off to 100 fathoms. The channel between Yanisada and the mainland is no deeper than . It rises to 100 fathoms again closer to the mainland, to comprise reefs around Sidero: Spitfire Rock, Pinnacle Rock, and others. Between Yanisada and Dragonada is about , and between the latter and Paximada, . North of Paximada are a few deeper, such as , before the deep. The chart depicts an uneven and rugged coastline, although its coves and headlands would probably not stand close GPS scrutiny. There are no geometric lines; the depths are sinuous and full of rocks projecting to the surface. There are only a few localities on the coast of Crete where ocean-going vessels have a clear and predictable road into a harbor. Around the Dionysades those were Sitia, and on the other side of Cape Sidero, Itanos, but the latter has been derelict for many centuries, with the harbor filled in to become a beach. ==Reference maps==