Mission help

I've got a mission that asks me to get 2 broken shards. I've got 1 but now even though it is showing on the map the location of the other, there's no sign of it. No black creature thing to shoot at to retrieve it and my meter doesn't bleep.

I ran out of time and let the tide turn so aborted the mission. Reassigned it on a fresh tide but still no sign of it. Am I doing something wrong? Not completed the bomb mission yet and wondering is this holding back progression?