You can get under the world without any special tricks from Utgarde Catacombs. I did a search, but it didn't turn up anything that wasn't about getting out of the Utgarde instances (Well, OK, I did find another, different hole in Utgarde which has since been patched,) so I apologize if this is a repost. I honestly couldn't find anything.
Facing towards the entrance to Utgarde Keep (the southern-facing dungeon at Utgarde,) the entrance to the Catacombs is on your left, under a large skull-shaped rock.
https://i.imgur.com/drreM.jpg
Immediately inside the cave entrance, between the two rock pillars chained together, look to your left
https://i.imgur.com/4SS6G.jpg
A nice big triangular hole in the wall. Hop up the slope, climb through, and you're under the world. I recommend flying upwards a bit to get over the spiraling tunnel that leads down into the Catacombs proper, as going downwards has a tendency to clip you into the tunnel and dismount you. If you're looking to avoid water, you'll want to exit off to the East. The bridge leading across the bay conceals a gap in the water that you can fly safely through.