The disciples, including Peter, had participated in the astonishing miracle that Jesus performed when he fed thousands of people although he only used five loaves and two fish. The impression is given that each of the disciples had a basket each of food after the people had been fed (there were twelve baskets), and that the disciples still had the baskets when they were told by Jesus to go into the ship (after all, he told them to go immediately). So we can imagine them crossing the sea with the visible signs of Jesus’ power beside them in the boat.
There is something new here, I think, and that is that Jesus sends them away in the boat without him. Of course, it might not have been a problem for some of the disciples to be in the boat without Jesus because they had been fishermen on the sea and were familiar with it. Others of them may not have been so confident, although we cannot say so for sure. So there may have been anxiety on the part of some at this requirement from Jesus.
At the same time, there should have been a sense of anticipation. After all, on the previous occasion when they crossed the sea, even in a storm, they had seen Jesus do something incredible with the deranged man of Gadara when he was delivered from the oppressive demon possession that he was under. And when they returned across the sea from Gadara, they had seen Jesus heal the diseased unnamed women and raise to life the daughter of Jairus. Although Jesus was not with them in the boat when they left on this new occasion, he had told them that he was coming later (he had sent them before him). Surely they must have wondered what he would do when he joined up with them again.
No doubt, they made some assumptions about how Jesus would rejoin them. Perhaps they thought that he would come later in another boat or maybe they expected him to walk round the shore. What is certain is that they did not anticipate that he would come in the way that he did. Assumptions do not prepare us for divine surprises, and if there is one attitude that disciples should not have, it is to assume what Jesus will do to them or through them or in them.