He is Life
On the last and greatest day of the festival, Jesus stood and said in a loud voice, “Let anyone who is thirsty come to me and drink. Whoever believes in me, as Scripture has said, rivers of living water will flow from within them.” By this he meant the Spirit, whom those who believed in him were later to receive. Up to that time the Spirit had not been given, since Jesus had not yet been glorified.
This took place during the annual Festival of the Tabernacles which lasts 7 days. By the last day of the festival, the crowds would have been sufficiently watered and satiated and whatever thirst their bodies may have had would have been certainly satisfied.
He is talking about spiritual thirst of the soul which is not satisfied by physical refreshment.
Even today this thirst of the soul exists for those who have tried to find satisfaction and fulfillment in the offerings of the world but, like drinking salt water, their thirst and dissatisfaction only grows.
To these, like the Samaritan woman, Jesus offers the promise of living water so abundantly that it will flow out of them.