The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com

3. And on account of the going of the prânas.

Scripture states that, when a new body is obtained, the prânas also go (from the old body to the new one). Cp. 'When he thus departs the (chief) prâna departs after him, and when the prâna thus departs all the other prânas depart after it' (Bri. Up. IV, 4, 2), and similar passages. Now this going of the prânas is not possible without a base; hence we infer that water also--mixed with parts of the other elements--goes (from the old body to the new one), serving the purpose of supplying a base for the moving prânas. For the prânas cannot, without such a base, either move or abide anywhere; as we observe in living beings.