Ken,
I don't know at what level there might be disagreement. WLC there says: "sign and seal of"... remission/forgiveness of sins. A sign doesn't have the same relationship to something prior as fruit does. I can put up a sign on an empty lot that says, "home of Brewster," and then build my house. That sign is not technically a fruit of the house, although it has a very real relationship to it, even before the thing is built.
A "fruit" would be something that is a product of something prior. We don't simply point to a baptism, and say "there's the fruit of that man's repentance." Only, I'd say, in an attenuated sense could be said that repentance produced a Christian baptism.
On the other hand, given baptist presuppositions, I think that such a connection demands less demonstration, because it is often said among them that "baptism is the first act of obedience of the Christian."
Again, I go back to the Jordan. John is speaking (by my rights) to members of the church, OT style. He's calling them away from their selfish, backsliding ways. As the result of a will to adopt the stance of a penitent, the person is baptized. That, it seems to me, shows a clear connection between the repentance, and the willingness to be washed clean by the water. John is saying, in other words, "Show your repentance by getting washed outwardly, and God will wash your hearts." It's a very clear connection.
I don't hear us (PBs) talking to the unsaved this way: "Come to Christ, and be baptized as a fruit of your repentance." Nor, do I hear us saying to the newly converted, "Come be baptized, as a result of your repentance." It seems to me that we adopt a more "Pauline" expression of a gospel call, and a call to be baptized more along the Pauline declarations regarding union with Christ, esp. his death.
Just look at Acts 19:3-5. Judge that situation and the words Paul uses with his words to the Philippian Jailer, 16:31. No mention of baptism there, although it occurs just after. And doesn't Paul say how few people he personally baptized, 1Cor.1:14? If baptism was so closely associated with the gospel call and repentance, I would think he must have baptized far more than he did. Baptism does not figure much in the gospel call to people who were outside the church. But Paul DOES mention it in the synagogue in Pisidian Antioch, Acts 13:24.
Baptism is a "sign" stating (as one part) that God remits/forgives sin. He does that spiritual work for those who have faith and repentance (conversion). The Mk.1:4 reference establishes a connection between baptism, repentance, and remission. But to say that the relation is best described as "fruit" seems a stretch.