Skyshard can be found on a boat, at the dock of Lunar Fang Docks POI. These docks are on a side of a river that connects central Cyrodiil and the southeastern lake. Do not try to swim in this river because the fish will eat you alive. Dying from fish is actually a good thing because you get an achievement “Slaughtered” when they defeat you. Upon reaching the docks you can notice many Lunar Fangs around it. These are hostile creatures that turns into Werewolfs as they detect danger. When they turn their health bar fills up. Just because of this it is pointless to burst damage them straight from the start. In order to reach the collectible you need to jump from the docks above on top of the boat.
Lunar Fang Docks in the distance.
Jumping into the river, in order to swim to the collectible, is not that smart … but it does yield an achievement.
Clear the docks, them jump into the boat.