In the bible milk is referred to as the beginning stages of learning about the word God. Hebrews 5:13 " For everyone that useth milk is unskillful in the word of righteousness : for he is a babe." Milk nourishes the body and helps it to grow. The more you drink the more you grow. 1 Peter 2:2 " As newborn babes,desire the sincere milk of the word, that ye may grow thereby". Infants thrive on milk because it is essentially the only thing they can digest. Once an infant gets older food is given in stages. This is equivalent to how we should be as we learn more and more about His word. Learning the word of the Lord is a growing process and we will mature the more we yearn for the word. The word of the Lord will also strengthen us if we are weak. Isaiah 40:31 " The spiritual milk of the word is an excellent restorative for a consumptive,wasted Christian: "But they that wait on the Lord shall renew their strength."
In the bible water is the first mention of life, and this life comes from water. Genesis 1:20 "And God said,Let the waters flow abundantley the moving creature that hath life,and fowl that may fly above the earth in the open firmament of the heaven." Water is essentially the major component of anything that physically contains liquid. This of course includes milk. Without water milk would utimately dry and of no substance. Without water we would die. In Genesis 2 the perfect conditions of the Garden of Eden are watered by a river. Without water the garden would have died along with the plants,animals and human beings. The river in a sense is dipicted as the life that Christ gives His children through the Spirit of God.
So the phrase "Don't trade milk for water" actually means that in order to understand the word of God we can't drink the milk if we have not been born (again). John 3:5 "Except a man be born of water and of the Spirit he cannot enter the kingdom of God." In this verse a water birth (physical) comes before the Spirit birth (spiritual). The realistic view of this can be compared to a woman in the natural can't give birth until her water breaks. We cannot be spiritually reborn until we have naturally been born. You have to recieve the Lord God as your personal Lord and savior in order for Him to work his miraculous wonders in and through you. 1 John 5:12 " He that hath the Son hath life; he that hath not the Son of God hath no life". Without water there is no life: Without the Lord Jesus Christ there is no eternal life.