The flow of water is vital for both physical and spiritual birth. Lastly, being born is bloody business. The placenta that supports the life of the pregnancy is bathed in blood. After the birth is completed, this blood flows quite freely until the womb contracts. Blood is important to life because it takes away the body's waste products and brings vital oxygen and nutrients to the body's tissues.
Spiritual life is bloody business too.
Christ had to die. His blood was shed to take away our spiritual waste called sin and to bring us the vital spiritual nutrients of forgiveness, life and salvation. But when they came to Jesus and found that He was already dead, they did not break His legs. Instead, one of the soldiers pierced Jesus' side with a spear, bringing a sudden flow of blood and water. We see this in physical birth.