Greg Gilbert writes about a professor repeatedly using "what may be the all-time worst argument for immersion."
It goes like this: In the story of Philip and the eunuch, the text uses the Greek words for "to go down [into]" and "to come up [out of]" water. Going down into the water and coming back up—that's immersion.
Well, no. That's a bad argument. Here's what the text says (Acts 8:38-39):
And he commanded the chariot to stop, and they both went down into the water, Philip and the eunuch, and he baptized him. And when they came up out of the water, the Spirit of the Lord carried Philip away, and the eunuch saw him no more, and went on his way rejoicing.
Now if "went down into the water" and "came up out of the water" refer to the baptism itself, are we to understand that both Philip and the eunuch were baptized? The text, after all, says that "they both went down into the water" and that they both came back up. Surely the text is not saying that both were baptized. Also, look carefully at the text's sequence; it's very precise. They went down into the water, he baptized him, then they came up. If "went down" and "came up" refer to the immersion itself, then we're left with the ridiculous picture of Philip pushing the eunuch under (presumably going under himself, too), then immersing him while they're both under water, and then both of them coming up together out of the water. What sense does that make? It's much easier to understand "went down into the water" as referring not to immersion, but to stepping down into whatever body of water the eunuch saw. Then Philip baptized him, and then they both stepped out of the body of water.
Update: To clarify, I (and Greg) hold to credobaptism by immersion as biblical.