The London Baptist Confession of Faith reads (Ch. 29:4) -
Immersion, or dipping of the person in water, is necessary to the due administration of this ordinance.In contrast the Westminster Confession of Faith (Ch 18:3) reads that -
"Dipping of the person into the water is not necessary; but baptism is rightly administered by pouring or sprinkling water upon the person."Which I think is properly interpreted that baptism is rightly administered (not only by dipping) but also by pouring or sprinkling, though the grammar seems ambiguous.
There are two concepts that Baptists often point to in their argument for immersion only. One of them is the meaning of the word βαπτίζω, which Strong's does (in their favor) write means "to immerse, submerge; to make whelmed (i.e. fully wet); used only (in the New Testament) of ceremonial ablution, especially (technically) of the ordinance of Christian baptism." Their other argument is that if baptism is a union with Christ's death and resurrection, then submersion in the water and raising up out of water rightly represents Christian union with Christ, and pouring or sprinkling does not.
I'd like to deal with the first reason, from Mark again. I love Mark.
In doing so, I'm also arguing against Strong's interpretation as well, but hey, if Scripture speaks, then it doesn't matter what everyone else says =).
7:1 Now when the Pharisees gathered to him, with some of the scribes who had come from Jerusalem, 2 they saw that some of his disciples ate with hands that were defiled, that is, unwashed. 3 (For the Pharisees and all the Jews do not eat unless they wash  their hands, holding to the tradition of the elders, 4 and when they come from the marketplace, they do not eat unless they wash.  And there are many other traditions that they observe, such as the washing of cups and pots and copper vessels and dining couches. )I need to include the footnotes here too, because they show us what's going on in the Greek -
 7:3 Greek unless they wash with a fist, probably indicating a kind of ceremonial washingAnd this argument flows straight from the footnotes. I notice that the NIV doesn't even have footnotes remarking the meanings of the words! Let me just make another plug for the ESV. =)
 7:4 Greek unless they baptize; some manuscripts unless they purify themselves
 7:4 Some manuscripts omit and dining couches
verse 4, footnote 2: "unless they baptize" - If we check the greek, we find that the verb for baptize is actually in the text. So the Pharisees and the scribes were baptizing their hands (with a fist!). Ok great, that certainly could include immersion.
But the next sentence I think pretty clearly rules that out -
And there are many other traditions that they observe, such as the washing of cups and pots and copper vessels and dining couches.We find that they did not only baptize their hands, but also their cups (ok that's doable), pots (difficult, but still doable), copper vessels (seems to be even bigger than pots, getting a little difficult now), and dining couches (well near impossible to be immersed without a great deal of effort).
I will readily concede that there exists a textual variant that dining couches were not originally included, but that seems to be the easy explanation out (and contrary to a basic premise of textual criticism, which was that the more difficult reading was more likely). But if dining couches were there (and that seems very likely), then it would seem that baptizo cannot mean exclusively to immerse, but must include a broader semantic meaning which allows for dining couches to be "baptized."
So that's my argument for a wider semantic range to "baptize" than simply "full-immersion"
This is not to say that the second argument (imitating Christ) may have credibility, but only that I don't find the first one conclusive. Of course, I have an argument against the second one too, but not this post.