Jesus answered, “Everyone who drinks this water will be thirsty again, but whoever drinks the water I give them will never thirst. Indeed, the water I give them will become in them a spring of water welling up to eternal life.”
Jesus continues to speak obliquely. Just as water is essential for life itself, so the spiritual water that Jesus gives is essential for spiritual life. What is this 'spiritual water'? It is nothing less than the Holy Spirit himself. He comes with belief in Jesus as the only way of salvation. He comes with repentance and a change of direction. He comes with commitment. It is impossible to believe without the Spirit and impossible not to receive the Spirit if one truly believes.
But we keep on getting thirsty and have to drink water every day. The Holy Spirit is self sustaining like a never-ending spring that keps welling up.