Skyshard is on the water, next to corals. This part of the map is called Coral Tower. You need to be on a certain quest that lets you climb the Coral Tower through Coral Tower Tunnel.
This quest is called “Between Blood and Bone“. You can accept this quest upon successful completion of “A Graveyard of Ships” quest. Get Graveyard of Ships from Captain One-Eye. You can find Captain One-Eye in a small camp, next to the road that leads to center of The Lost Fleet (Lighthouse POI).
Note: In order to reach northern part of Coldharbour you need to complete the main quest “The Army of Meridia”.
Update 1: Just west of solo dungeon Wailing Maw map mark there is a large wooden pole. You can use it to climb toward the platform with the shard on. Keep on mind that you have to make one final jump from the pole’s top in order to reach the platform.
Start working on The Lost Fleet quest chain with mission “A Graveyard of Ships”.
Map location of Captain One-Eye that gives the quest.
The second quest in the chain is “Between Blood and Bone”. It is also given by Captain One-Eye but on a different location.
This quest lets you enter the Coral Tower Tunnel.
Coral Tower Tunnel entrance map location.
When you exit the tunnel, turn right and keep following the small passage.
Soon the Shard reveals itself.
Update 1: Wooden Pole west of solo dungeon Wailing Maw.
Update 1: The final jump.