In my own understanding water baptism is not necessary, as God don't care your surface but your heart (i.e. God don't care the burnt offerings), and water baptism is just a procedure. The thief that died with Jesus went to heaven without been baptized.
However my pastor told me that when possible, always baptize, else he is not totally sure..... So if conditions allows, why put in any doubt on your salvation
For Lutherans it comes down to understanding Baptism as Gospel rather than Law. God has attached certain promises to Baptism, hence the Lutheran understanding that a Sacrament is defined as a visible element comprehended with God's word. If Baptism were merely water it would mean nothing but it is God's word joined together with the water that makes Baptism of any significance--which is why Baptism is efficacious to accomplish what it conveys.
That doesn't make Baptism "absolutely necessary", as though one cannot be saved apart from baptism, because again it is God's word that is efficacious. Which is precisely why the thief on the cross is saved because of the word which Christ spoke to him, "Truly I tell you, today you will be with Me in paradise."
It is that same saving word of Gospel that is found in the Sacrament of Baptism, it's why Baptism is efficacious to accomplish that which Holy Scripture says of it. Mere water cannot unite a sinner to Christ, but we read in Romans 1 that the Gospel is the "power of God to save all who believe" and in Romans 10 "faith comes by hearing and hearing by the word of God". It is through the Means of Word and Sacrament that God creates and grants faith to us in order to believe and trust in Christ.
Since we are powerless to turn to God and to make ourselves righteous by our own efforts (Ephesians 2:8-9) we must rely on the external word of God. We have been told where God indeed operates to accomplish what He will--in His Word and Sacraments--outside of which we have been told nothing.
This is why Lutherans always point to the external Means of Grace, God's Word and Sacraments, not to our own strength, efforts, and works. We are powerless to save ourselves or accomplish anything toward our own righteousness, but are instead helpless and hopeless sinners relying upon the mercy and grace of God alone, on Christ's account alone, alone through faith which is from God alone.
-CryptoLutheran