We all need water to live. On average a person cannot live more than three days without drinking water. This is not to mention all the other uses we have for water such as cleaning and bathing.
Is it any wonder then why our Lord instituted baptism using water? The water of baptism is spiritually life-giving, just as plain water sustains our physical life. The water of baptism cleanses us from sin, just as we use water to bathe our bodies. Since water is so basic to human existence it's almost always readily available for baptism.
When the people of Israel left Egypt they had difficulty finding drinking water out in the wilderness on more than one occasion. So, God had to provide water for them. This He did through Moses. God made the bitter waters of Marah sweet (Ex. 15:23-25), and miraculously made water gush out of solid rock (Ex 17:6, Num. 20:8-11.) The people of God needed this water to live.
You and I need the water of baptism to live. St. Peter says "Baptism now saves you" (1 Peter 3:21). St. Paul says baptism is, "the washing of regeneration and renewal of the Holy Spirit" (Titus 3:5). And Jesus says, "Whoever believes and is baptized will be saved" (Mark 16:16). Thanks be to God that he gives water to sustain our earthly lives, and the water of baptism to grant us eternal life.