It clear in the Bible when someone puts faith in Jesus and is baptized, the location chosen is always one with plenty of water and by going down to it.
I am comfortable with the assumption that immersion was common, even standard, mode of baptism. But historically we know that pouring was a perfectly acceptable mode of baptism, at least when there was an insufficient quantity of water. We see this in the early Church document known as the Didache (written sometime between 60 and 120 AD), which provides the most ideal baptismal conditions: immersion in cold running water; but that this was not a hard requirement, the ancient Church wasn't legalistic or exceptionally picky about this. If there wasn't any cold running water, it was fine to use standing water, even warm water if cold wasn't available, and if there wasn't enough water to immerse in, then pouring was perfectly fine.
And that's simply how it's been the last two thousand years, from New Testament times until now. The mode of baptism has never been of extreme importance--what mattered was that it was done in accordance with Christ's command: the Church is commanded to baptize in the name of the Father, Son, and Holy Spirit in Matthew 28:19. There have been ideal situations, generally it has been ideal for a pastor to perform the baptismal rite, and as part of a service, preferably on Easter Sunday as a way to provide a grand welcome to the baptized in the Church of the Risen Christ. But if that doesn't happen, as long as it is Christian baptism, it's baptism. Who performs the baptism isn't what matters, the mode isn't what matters, the type or quantity of water doesn't matter--what matters is that it is Christian baptism. This is why the historic churches fully accept the baptisms of other Christians, regardless of denomination--insofar as it was a Christian baptism then that's what matters.
In modern times a small number of Christians have become very rigid about baptism, based upon their own particular interpretations and traditions. Which is made weirder when one also realizes that those who are most rigid, or even legalistic, about baptism also don't believe baptism is anything more than a symbolic act, that it is an empty religious ritual that doesn't actually do anything except get a person wet.
Here's a question worth asking yourself: Why do you feel it necessary to insist that descriptions of baptism in the New Testament are prescriptive--especially when it actually isn't all that clear how the baptism was being done. If we add up all the times someone is baptized, starting in the Acts of the Apostles (since we are talking Christian baptism, not the baptism of John the Baptist, not Jewish tevilah), we actually don't get a clear description. We get, at best, a handful of rough descriptions, but mostly no description is ever given.
I am comfortable assuming immersion was probably how it was usually or generally done, not because the Bible makes that clear (it doesn't), but because extra-biblical early Christian sources--like the Didache mentioned above--say that.
No where in Scripture is immersion-only baptism taught. Such a view is not, and indeed cannot, be derived from Scripture; rather it is a modern and man-made tradition imposed upon Scripture.
I know that saying this will come off as offensive. As someone raised in an immersion-only church tradition, there were a lot of things that I just took for granted as "biblical". The argument that "baptize" means "immerse" and can only mean "immerse" was one I used to use myself when I was younger, because it's an argument I learned from pastors, youth pastors, Sunday School teachers, etc. So it's the argument I also used. And because of that, anything different than that was what I could safely accuse of being "traditions of men", and remind myself that Jesus condemned the traditions of men because tradition meant ignoring the word of God. Back then one of the most offensive things that I could think being told is that what I was practicing was just a human tradition. Which is why I know me saying this about immersion-only baptism will offend some, because it would have offended me back when I subscribed to an immersion-only view of baptism.
But offense isn't my intent. The intent is to provoke deeper reflection.
-CryptoLutheran