Everything before the cross was the Old Testament. If you want Old Testament proof of water baptism, look at John the Baptist, he was under the law as well as Jesus.
Galatians 4:4 But when the fullness of the time was come, God sent forth his Son, made of a woman, made under the law,
There are those out there that are trying to do away with certain practises namely water baptism. It is necessary that you continue reading your New Testament, there it will prove that water baptism is still valid and it's a part of obedience that the LORD and the Apostles have laid down for us to follow.
Many use this scripture for their argument.
Ephesians 4:5 One Lord, one faith, one baptism,
They contend that since there is only one baptism there can't be two. However, the argument becomes weak when you introduce other passages
Hebrews 6:2 Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgement.
Here in this verse we can see that baptisms is in the plural.