Immersion is absolutely specified in scripture. The Greek word that is transliterated as baptism means immersion. There are specific Greek words for sprinkling and pouring, and they are not used in reference to water baptism in the New Testament.
Have you ever read the Didache? It was written around the year 90 AD and contains some of the the earliest Christian instructions.
Here is the chapter on Baptism:
Chapter 7. Concerning Baptism. And concerning baptism, baptize this way: Having first said all these things, baptize into the name of the Father, and of the Son, and of the Holy Spirit, in living water. But if you have no living water, baptize into other water; and if you cannot do so in cold water, do so in warm. But if you have neither, pour out water three times upon the head into the name of Father and Son and Holy Spirit. But before the baptism let the baptizer fast, and the baptized, and whoever else can; but you shall order the baptized to fast one or two days before.
http://www.earlychristianwritings.com/text/didache-roberts.html