The Dice Snake is a strongly aquatic snake associated with natural waterside habitats. Donau-Auen describes it as a skilled swimmer and diver whose hunting takes place in water. Its patterned back supplies the English name: dark markings form a roughly chequered arrangement on a grey or olive background. The relevant cruise association is the Austrian floodplain east of Vienna towards Hainburg, not the entire Danube route. The park's more recent snake overview calls it very rare and difficult to see. Visitors should therefore treat a sighting as a possibility on a specialist backwater walk rather than a routine expectation from a ship. Species background source: https://www.donauauen.at/wissen/natur-wissenschaft/fauna/wuerfelnatter-natrix-tessellata-tessellata

Danube
Austria: Donau-Auen floodplains east of Vienna towards Hainburg, on Vienna–Bratislava reach; optional shore/backwater excursion, not Passau or Budapest. Viewing: Warm months; watch natural banks for basking or swimming snakes.

Aesculapian Snake
European Pond Turtle