After evening falls, the disciples board a boat and head across the Sea of Galilee toward Capernaum. Darkness comes, a strong wind raises the sea, and they row twenty-five or thirty stadia before seeing Jesus.
The details describe strenuous travel rather than a quiet crossing.
Boats were essential for fishing and transportation around the Galilean lake. Its position below sea level and surrounding topography can contribute to rapidly changing wind conditions and rough water. A stadion was roughly 185 meters, so twenty-five to thirty stadia represents approximately 4.5 to 5.5 kilometers, or about three miles, allowing for normal variation in ancient measures.
John's distance is an approximation, not a modern navigational coordinate.
The geography helps readers picture the disciples well out on the lake, rowing in difficult conditions after dark. Jesus' appearance therefore occurs where ordinary access to them would be impossible.
The setting heightens the sign without requiring exaggerated claims about uniquely supernatural Galilean weather: the wind and distance are ordinary realities surrounding an extraordinary encounter.