There are two possible reasons that the disciples marveled at what sort of man
Jesus is:
They did not yet understand
what sort of man
Jesus is, although they should have known from the Old Testament that what Jesus was doing here was exactly what God had done in the past.The disciples’ weak faith, which broke under trial, was now transformed and strengthened by Christ so as to marvel over his works—the first step towards confessing that Jesus is the Son of God (Matthew 14:33) and the Messiah (Matthew 16:16). For now, they only view Jesus as more than a mere man.
Interpretation 1:
For now the disciples receive no clear answer to the question in their minds regarding what sort of man
Jesus is. They marvel, even though by now anyone acquainted with the Tanach or Old Testament should have had to recognize that what Jesus was doing here was exactly what God had done in the past.1
Interpretation 2:
The faith of the disciples had not been able to withstand the trial, but is now transformed and strengthened by Christ so as to marvel over his works. This is a first step towards confessing that Jesus is the Son of God (Matthew 14:33) and the Messiah (Matthew 16:16). For now it is only evident that they view Jesus as more than a mere man.2
27 Ces hommes furent saisis d'étonnement: Quel est celui-ci, disaient-ils, à qui obéissent même les vents et la mer?