Or wait for flight
I'm surprised so many things from earlier than 9.0 even work in Shadowlands at all, nevermind Zereth Mortis. Blizzard probably decided to wait for articles like this one to highlight stuff they need to lock down.
I was able to get to the raid entrance by getting to the top of the mountain on the edge of the zone with the help of the frog jumping world quest. Then used a goblin glider to get to a foothold on the other side. Then used a lock gate to advance to the main part of the island. I did find a pool of regular water (swimmable) so it does exist. lol
Everyone is always sleeping on Darkmoon Cannon when posts like this get made.
Huh. On the second day when Zereth Mortis was available, I just happened to look at my alt's unlearned Engineering recipes and saw the Gunshoes. I thought to myself how useful they would be for Zereth Mortis and how easy they are to craft. I guess others had the same idea!
Or they could have just let us fly since last time they gave us the ability to fly but the new zone was ground only so flying only applied to old zones we didnt play in any more. 1st time ever having to unlock flying twice in one expansion and conveniently locked behind 1 month of sub time in case flying makes us realize how lacking in content the new zone is so we would pay for a 2nd month before unsubbing
I just use the quest item that makes you teleport around to click the 12 robots on the cliffs. I wait and turn it in to chain use it to get close to other quests and if a rare spawns across the zone.
I am surprised these are never in any guides... I use "the original" extremely fast mount items from MoP: Keeping both Oddly-Shaped Horn and Tuft of Yak Fur in my bags (as I am still farming Zandallari Warbringers and MoP World Bosses anyway). With these, despite having just 3 charges, I can zoom through the zone very quickly, as they have a CD of just a couple of seconds.