
That's Jesus there in the back of the boat, lying down, just awakened by the panicked disciples.
Notice the two approaches to dealing with storms that Rembrandt captures in this painting. There are the disciples up at the bow of the boat, furiously trying to set the sails and right the vessel. And then there are the ones back at the stern. I think they realize just how hopeless their efforts will be against the storm - how absolutly futile it is to pull at those lines when you're up against the wind and the waves.
And they're almost certainly more afraid than they ought to be, what with the Son of God in their craft. But there they are, on their knees, looking to Him for help.
Where are you in this painting? Up at the bow, pulling and striving while the waves crash around you, threatening to sink your little boat? Or there with your Lord, crying out in whatever weak and broken way that you can that He alone is your rock, your refuge.