It should be no surprise that in this world fragmented by rising seas, travel by water is an essential means of transportation. Whether it is casks of olives and amphorae of wine from Armorica, timber and wool from Caledonia, or slaves and contraband from Uyghuria, anything that travels anywhere in the Drowned World does so by water. On a clear summer day in one of the busy shipping lanes, a captain can stand atop the sterncastle of his ship and see uneven lines of sails dotting the distances ahead and behind.
Although the level of technology throughout Drowned World is more or less comparable to that of our own late medieval period, shipbuilding and sailing have advanced much further, to a level that far exceeds the apex of sailing technology in our own world. Even the simplest types of craft, such as the open-hulled longships of the Northmen and the sealhide coracles that ply the waters between the Nethyian Isles, ride the winds and waves with surprising grace. The finest examples of sailing vessels - Hibernian schooners and elven wingships - travel with almost supernatural ease. Unlike so many other things in the Drowned World, however, it is not due to magic, but rather to highly evolved technology and skilled craftsmen.
Despite these technological advantages, seafaring in the Drowned World is still a dangerous undertaking. There are, of course, any number of meterological dangers: sudden storms, waterspouts and fierce cyclonic storms are common in the warm, shallow waters that cover much of the Drowned World, and the blue waters of the Northern and Western Oceans are notoriously treacherous. In the heat of summer, some of the shallower waters become so clogged with beds of kelp and sargasso that ships foul their rudders and become trapped. Many fall prey to orc marauders, or to the swarms of sahuagin that boil from the depths to overwhelm ships and slaughter their hapless crews. In the deep waters of the Northern Ocean, storm-battered ships can be split asunder by submerged icebergs, or attacked by raiding parties of Coelegath. Some vessels simply disappear into the foggy vastness.
Established shipping lanes are the safest places to travel, as they are generally patrolled by warships of lawful kingdoms such as Burgundia and Brittania. These safe routes are mostly confined to the Central Sea and the Gulf of Brittania, where the majority of the Drowned World's commerce is found. Other waters are far more dangerous. The Drowned Plains, which border Uyghuria to the north and the Gull's Neck to the east, are generally considered the most dangerous waters in the world, for they encompass the Lost Archipelago, a locus of smuggling and piracy, and also the Gish Delta, home to several powerful orc tribes, as well as a major artery of the Uyghurian slave trade.
If your party is to return to civilization, it must sail through some of these waters. Good luck, and godspeed...