An interesting question, in terms of why did God decide to save His children through water. 1 Peter 3:18-21 says, "For Christ also died for sins once for all, the just for the unjust, so that He might bring us to God, having been put to death in the flesh, but made alive in the spirit; in which also He went and made proclamation to the spirits now in prison, who once were disobedient, when the patience  of God kept  waiting in the days of Noah, during the construction of the  ark, in which a few, that is, eight persons,  were brought safely through the water. Corresponding to that, baptism now saves you--not the removal of dirt from the flesh, but an appeal to God for a good conscience--through the resurrection of Jesus Christ," (NASB-as is all following quotes).

A Long History of Baptism

The reference to Noah of course is the account of him, his wife, his three sons and their wives, being saved from the flood that destroyed the earth by taking refuge in the ark (Gen. 7:5). Peter reminds us that these eight people 'were brought safely through the water'. Flash forward to the children of Israel fleeing Egypt, on their way to the promised land. When they got to the Red Sea, God through Moses parted the Sea and the Israelites passed through the water on dry land (Exodus 14). Paul reminds the Corinthians that these were baptized by this passing through the water (1 Cor. 10:1-2).

In the days of John the Baptist, he was teaching those who were coming to him about baptism (Mark 1:4). He even baptized our Lord (Matthew 3:13-17). In Acts chapter 2, when those to which Peter convicted of their sins asked what must they do, Peter told them to 'Repent, and each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the Holy Spirit' (Acts 2:38). After Phillip had caught up to the Ethiopian eunuch, ' Then Philip opened his mouth, and beginning from this Scripture he preached Jesus to him.  As they went along the road they came to some water; and the eunuch said, "Look! Water! What prevents me from being baptized?"  And Philip said, "If you believe with all your heart, you may." And he answered and said, "I believe that Jesus Christ is the Son of God."  And he ordered the chariot to stop; and they both went down into the water, Philip as well as the eunuch, and he baptized him' (Acts 8: 35-38). Even Paul was baptized for the remission of his sins (Acts 22:16).

There is a long history of people being saved through water. So it is not a foreign idea that baptism is necessary for salvation under the law of Christ.

Baptism Only...Grace Only?

Again let's step back in history, knowing that the Old Testament was given to us as a tutor, leading us to Christ (Gal.3: 24-25). When the children of Israel invoked the wrath of God and He sent fiery serpents in among them to punish them. He provided, by His grace, a means by which they could be healed after being bit (Num. 21:6-9). But they had to do something in order to be healed; they had to look at the bronze serpent that Moses had made and put on a staff.

Our Lord said of Himself, ' As Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up; so that whoever believes will in Him have eternal life' (John 3:14-15). But wait...Jesus says one must only believe in Him to have eternal life? Yes, He does. In fact He says only 'believe' throughout this passage. So that means that repentance and confession are not necessary for salvation, since they are not mentioned here either, right? If you are teaching the 'ABC's' of salvation (the teaching that you must Admit you're a sinner, Believe in Jesus, and Confess that Jesus is Lord and you'll be saved), then A, B and C must be present in every passage about salvation in order for you to say that baptism can be excluded from the list. Clearly, the ABC's are not present in every passage. And neither is baptism. But that does not mean that is not authoritative because it is only mentioned in some passages and not others. God's word need only say something once for it to be authoritative (Isa. 55:11, Matt. 4:4, 5:18-20), as you would probably agree when it comes to the ABC's.

So, baptism alone? Certainly not. Just as those in Acts 2 repented and the Ethiopian believed and confessed, we must also do these things before being buried with Christ in baptism: 'Therefore we have been buried with Him through baptism into death, so that as Christ was raised from the dead through the glory of the Father, so we too might walk in newness of life' (Rom. 6:4).

Ok, then grace only? Not grace only either. God's grace is demonstrated by Him giving us a means by which we might be saved (Rom 6:23). But it does not come to us without us doing something. God's grace has given us the way of salvation, our responsibility is doing what He has commanded in order that we might receive that salvation (James 2:18-26).

But Why?

Why is so much time and energy put into trying to prove something that is not stated in the bible? Nowhere is the 'prayer of faith' (asking Jesus to come into your heart), or that 'baptism is an outward display of an inward faith' found in scripture as the means by which we are saved. These are commandments of men, which our Lord spoke against (Matt. 15:9). Rather, these arefound in scripture:

This is an awful lot of ink for something that is not really necessary for our salvation, especially given how precious each word of scripture is (John 20:30-31, 21:25).

So why baptism? Simply put, it is how we come in contact with the death of Jesus Christ (Rom 6:3-11, Col. 2:12). And that is so very important because it is the basis of our salvation. Jesus died for sin so that we don't have to (Heb. 10:10, 1 Pet. 3:18). God decided that this was the means by which He would save His children, and that should be enough for us to answer the question of why. We must trust in His wisdom and not our own...

1 Cor. 1:18-31

18 For the word of the cross is foolishness  to those  who are  perishing, but to us who are  being saved it is the  power of God. 19 For it is written, "I WILL DESTROY THE WISDOM OF THE WISE, AND THE CLEVERNESS OF THE CLEVER I WILL SET ASIDE." 20 Where  is the wise man? Where is the scribe? Where is the debater of this  age? Has not God made  foolish the wisdom of the  world? 21 For since in the wisdom of God the  world through its wisdom did not come to know God, God  was well-pleased through the foolishness  the message  preached to save  those who believe. 22 For indeed Jews  ask for signs  and Greeks search for wisdom; 23 but we preach Christ  crucified, to  Jews a stumbling block and to Gentiles foolishness,  24 but to those who are the  called, both Jews and Greeks, Christ the  power of God and the  wisdom of God. 25 Because the foolishness  of God is wiser than men, and the  weakness of God is stronger than men. 26 For consider  your calling,  brethren, that there were not  many wise according to the  flesh, not many mighty, not many noble; 27 but God  has chosen the foolish things of the world to shame the wise, and God has chosen the weak things of the world to shame the things which are strong, 28 and the base things of the  world and the despised God has chosen, the  things that are not, so that He may nullify  the things that are, 29 so that no  man  may boast before God. 30 But by  His doing you are in Christ  Jesus, who became to us wisdom  from God, and  righteousness  and sanctification,  and redemption,  31 so that, just as it is written, "LET HIM WHO BOASTS, BOAST IN THE LORD."