John 6:16, "When evening came, his disciples went down to the sea," is a simple statement of fact setting the scene for the next events in the Gospel of John. Here's what it likely means in context:

Setting the Scene: This verse serves as a transition from the feeding of the 5,000 (John 6:1-15) to the next miracle, Jesus walking on water (John 6:19-21). It bridges the day and the night and shifts the location.
Time of Day: "When evening came" indicates that the feeding of the 5,000 had taken place earlier in the day and now night was falling.
Disciples' Actions: "His disciples went down to the sea" tells us the disciples were near the Sea of Galilee (also known as Lake Tiberias) and they were heading towards the water. This would logically be to board a boat, as the following verses describe.
Implied Purpose: Although not explicitly stated, the implied purpose of going to the sea was to travel to the other side, likely towards Capernaum, which is mentioned later in the chapter (John 6:24). Jesus had withdrawn to a mountain alone (John 6:15), and the disciples were presumably following his earlier instructions or their usual routine to travel by boat to another location.
Contrast to Jesus' Location: This verse also creates a contrast between the disciples' physical journey by boat and Jesus' upcoming supernatural journey on the water. The disciples are in the natural realm, while Jesus is about to act in the supernatural.

In essence, this verse is a simple narrative element that sets the stage for the next episode in the Gospel story.