Did the disciples understand that Jesus had risen when they were at the Sea of Galilee?
No. Even after recognizing Christ in His resurrection body, the disciples had not fully grasped the reality of His resurrection. They still could not contemplate the whole idea that Christ had been raised from the dead, as shown when they were fishing on the Sea of Galilee.
Les explains that although the disciples had recognized Christ in His resurrection body, it still was not locked in on them. They had not been able to contemplate the whole idea of Christ being raised from the dead. So on the Sea of Galilee, they were fishing all night, and the Lord stood on the shore the next morning at breakfast time. Their lack of full understanding is evident in that setting.