What does John 6:18 mean?

Mark chapter 6 and Matthew chapter 14 provide many useful details about this particular incident. John intends His gospel to supplement those accounts, not to replace them. Among the details not listed here are that Jesus has been watching the men from the surrounding hillside and will eventually walk out on the water towards them (Mark 6:48), and that this is the incident where Peter briefly walks on water, as well (Matthew 14:28–32).

The terrain around the Sea of Galilee is shaped in a way which contributes to sudden storms. At the moment this incident occurs, the disciples' boat is right in the middle of the lake (John 6:19), which is only about 7 miles (11 km) long. According to Mark, the disciples are "rowing" against the wind. This means the storm they were experiencing was more or less exactly as described here in the gospel of John: a "strong wind." This was not a typhoon, or a hurricane-level squall. All the same, a small boat and wind-driven waves combine for an uncomfortable experience. It is worth noting, however, that the fear these men felt was because they initially thought Jesus was a ghost, not because they were afraid of sinking (John 6:19; Mark 6:49).