Mark 6:50-51

‘But he immediately spoke with them and says, “Be of good cheer. It is I. Do not be afraid.” And he went up to them in the boat and the wind ceased.'

Jesus immediately assured them that all was well. They had failed the test but they would learn from it in the future. It was at this time that Peter made his attempt also to walk on the water (Matthew 14:28). Then having assured them that it was really He, He approached the boat and clambered in. And to their surprise the wind immediately ceased. But note that this was the cessation of a contrary wind, not the stilling of a storm.'

It is I'. The Greek is ‘ego eimi' - ‘I am'. While Jesus simply meant ‘it is I' (this is the regular Greek for that), it is possible that Mark intends us to get the inference that He is the ‘I am', the God of the covenant (Exodus 3:14). That is why they do not need to be afraid.