The sea is the deepest truth of God, and standing on the far side of it are the people fixed on the outside, unable to see how the awakened center crosses that depth without any visible vessel. The single boat carries the meditative faculties while the higher self does not travel by that outward means, showing that the kingdom is reached inwardly and silently, not by any craft the surface mind can point to. Those left staring at the shore look for a mechanism where none exists.
What is drawn here is the search conducted from the outside for something that only opens within. The deep truth is crossed in stillness, and the parts of the self that meditate go on ahead while the literal-minded remain puzzled at the edge of the water.
Sea: the deepest truth of God, the depth crossed only inwardly
Boat: the outward means the surface mind expects, insufficient for the inner crossing
Disciples: the meditative faculties of the self that go ahead across the deep truth