The Dothraki refer to the ocean as the poison water; they dislike, mistrust and fear it. Water that a horse cannot drink is something foul and the heaving great grey-green plains of the ocean fill them with superstitious loathing. Nor do they like the fact that a horse cannot cross the poison water on its own legs.
A Clash of Kings
On the day Daenerys Targaryen’s three ships lifted anchor at Qarth, she thinks to herself that you'd have thought they were all sailing to hell instead of Pentos. Irri and Jhiqui clutched the rail desperately and retched over the side at every little swell. Even her brave young bloodriders stared off at the dwindling coastline with huge white eyes, each of the three determined to show no fear before the other two.
Dany’s tiny khalasar chose to remain below decks throughout the passage, preferring the company of their nervous horses to the terrifying world about the ships. When a sudden squall enveloped them six days into the voyage, she heard them through the hatches, the horses kicking and screaming, the riders praying in thin quavery voices each time the Balerion heaved or swayed.
A Dance with Dragons
When Daenerys Targaryen surmises that Xaro Xhoan Daxos's thirteen ships may be enough to carry them to Westeros, Irri and Jhiqui exchange a look. Irri tells Daenerys the poison water is accursed, horses cannot drink it. Daenerys promises them that she does not intend to drink it.
No horse can cross the poison water.—Khal Drogo