The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
2. But on account of (water) consisting of three (elements) (the soul is enveloped not by water merely; the latter alone is, however, mentioned) on account of preponderance.
The word 'but' disposes of the objection raised.--Water consists of three elements, as we know from the scriptural statement regarding tripartition. If, therefore, water is admitted to originate (the new body) the other two elements also have necessarily to be admitted (as taking part in the origination). The body moreover consists of three elements, as the effects of the three, i.e. fire, water, and earth, are observed in it, and further as it contains three materials, viz. wind, bile, and phlegm 3. Being such it cannot originate from mere water, the other elements being left aside. Hence the term water made use of in the scriptural question and answer refers to the fact of water preponderating,
not to its being the only element. As a matter of fact we see that in all animated bodies liquid substances such as juices, blood, and the like preponderate.--But we likewise observe in bodies a large amount of earthy matter!--True, but the amount of water is larger than that of any other matter. Moreover, liquid matter prevails in that which is the seed of the body. Further, we know that works (karman) constitute the efficient cause for the origination of a new body, and (sacrificial) works such as the agnihotra, &c., consist in the offering of liquid substances such as Soma, butter, milk and the like. Thereby also the preponderance of water is established. And on account of that preponderance the word 'water' implies the subtle parts of all the elements which constitute the seed of the body.
104:3 The last of which only is of prevailingly watery character.