There is a small underground sea that behaves as a subterranean version of the Panama Canal called the Black Way and the Dark Elves pretty much have 100% control over it. The Dark Elves thus can raid the west coast of Lustria pretty easily and they can sail to Warhammer Asia much easier than the other Old World fashions. I asked around the Dark Elf forum. I could not find a map that marks where the east and west entrances to the Black Way are, so I guess fluff writers can put them anywhere they choose. The Black Way has a lot of rogue sea monsters, that make sea travel more dangerous than usual, but no major Warhammer army (not Cathay, not Chaos, not the High Elves not the Empire) has contested the Dark Elves' control of the Black Way. Few outside of Naggaroth are aware of it. In my fluff, the Klodorex alliance has discovered the Black Way, but Lustria doesn't control a navy, so they aren't sure how to close it off to Dark Elves. Slann are contemplating what to do about it, as are Skinks who are now trying to capture Old World ships intact and/or reverse engineering the ship designs. As of now, their efforts to create a Lustrian Navy are laughable failures. Since it's Skinks doing the sailing, most of the would-be sailors survive when their vessels keep sinking. Note that the Skinks are making quixotic plans for ship-to-ship combat. It has not occurred to them that they may have to fight Black Arks (if you are fluff challenged, Black Arks are fortified stone towers that magically float and move at the speed comparable to warships). When Malekith launches a full scale invasion of Ulthuan he typically rams a bunch of Black Arks into the coastline to serve as his forward base. When there isn't a full scale invasion of Ulthuan, the Black Arks are sailing solo to go serve as linch pins for invasion fleets raiding human lands and Lustria. There are dozens of Black Arks, and after centuries of bitter warfare, the High Elves in official canon sank one. A couple times the High Elves have forced Black Arks to retreat. No one else has come close to defeating a Black Ark at sea. Few apart from the High Elves are aware that the Dark Elves have Black Arks because the Dark Elves rarely let witnesses to their floating towers survive to tell the story.
https://warhammerfantasy.fandom.com/wiki/Black_Ark apparently it got mentioned in the 4th, 5th, and 7th edition high elves armybooks, and the 8th ed dark elves one. there was also the Talon of Agony, apparently lost to a tidal wave off Cathay. (sounds like a callout to the 2nd Mongol invasion fleet destroyed off Japan.)
I like this discussion but I am OCD about categorization. I have a started a new thread on Dark Elf Black Arks.