The "rivers of living water" is a metaphor that represents the life that Jesus provides for those who are spiritually "thirsty." Alternate translation: "spiritual life will flow like rivers of water" (See: [[rc://en/ta/man/jit/figs-metaphor]])
Here the stomach represents the inside of a person, specifically the non-physical part of a person. Alternate translation: "from inside of him" or "from his heart" (See: [[rc://en/ta/man/jit/figs-metonymy]])