Casting out demons, healing people, teaching with authority: the disciples had seen (or heard) all of these. Then Jesus demonstrated His command of creation itself:
Mark 4:39 When Jesus woke up, he rebuked the wind and said to the waves, ‘Silence! Be still!’ Suddenly the wind stopped, and there was a great calm.
This was something the disciples had neither seen nor anticipated:
Mark 4:41 The disciples were absolutely terrified [or amazed, according to Matthew]. ‘Who is this man?’ they asked each other. ‘Even the wind and waves obey him!’