In the middle of a complex and difficult reality, a deep longing for the safety and familiar landscapes of childhood can suddenly surface. David experiences an intense craving [מצודת ציון, ביאור שטיינזלץ]. Having grown up in Bethlehem, he remembers the good water of his hometown, which is located not far from his current camp in the Valley of Rephaim [ביאור שטיינזלץ]. Even though he could easily drink the water collected in the nearby desert pools, he longs specifically for the water of his youth.
This deep yearning is rooted in basic human nature. People naturally adapt to the air and water of the place where they are raised, finding comfort and health in them, while an unfamiliar environment can sometimes feel harsh or even harmful. To illustrate this idea, there is a story of a queen who fell ill after marrying and moving to a foreign country. Her doctors determined that she would only recover if she could bathe in and enjoy the waters of her native land [רש״י].
When David asks out loud who will bring him water to drink, he is not issuing a military order or commanding his men to undertake a mission. He is simply speaking innocently from his heart, voicing a quiet, inner wish and a profound sense of homesickness [מצודת דוד, ביאור שטיינזלץ].