Expositor's Bible Commentary Commentary Mark 4:37

"And there ariseth a great storm of wind, and the waves beat into the boat, insomuch that the boat was now filling." — Mark 4:37 (ASV)

The geographic location of the Sea of Galilee makes it particularly susceptible to sudden, violent storms. It is situated in a basin surrounded by mountains. Though at night and in the early morning the sea is usually calm, when storms come at those times, they are all the more treacherous. The storm mentioned here drove the waves into the boat so that it was being swamped.