1 John contains more evidence that John’s baptism was the only Messianic water baptism known to the apostles. There simply was no other.
This letter was written by the apostle John to counter the false teaching of the early Gnostics. They taught a distinction between Yeshua and the Messiah, making Yeshua the Messiah only after the Spirit descended on Him after being baptized by John (contrary to Luke 2:11). However, they also said that the Messianic Spirit left Yeshua while He suffered on the cross so that He did not die as the Messiah. He lost His Messiahship before He died.
In John’s endeavor to expose Gnostic teachings he mentioned the "water" as part of the testimony which proves Yeshua’s Messiahship, 1 John 5:6-8, in conjunction with the blood and the Spirit.
Different ideas have been expressed as to what is meant by "water and blood," including the reference to "blood and water" (note the reverse sequence) in John 19:34.
It appears from the context that John is speaking specifically about Yeshua entering His office as Messiah. John wants to establish the water of Yeshua’s baptism, the blood He shed in His suffering, and the Spirit as crucial testimony that He truly is Messiah. Here is the Heaven sent Redeemer who came into His redemptive service through the water of John’s baptism and through the shedding of His blood, Yeshua the Messiah.
Since this water is so important someone might think all disciples must also be baptized. But we must remember that Yeshua’s "water" was R. John’s baptism. Traditional Christian thought does not believe John's baptism is Christian baptism nor does it provide salvation in the New Covenant age.
Notice the apostle John did not try to distinguish this water from some other supposed newer command by Messiah. He did not call this water of 1 John 5:6-8 "John’s water," or "John’s baptism" he simply calls it "the water," apparently unaware of any newer water purification ritual commanded by Messiah.
It was John’s baptism in which the Messiah fulfilled all righteousness of the Torah and Prophets. John's baptism was a Jewish rite which falls in the category of a work of righteousness and is not crucial to God after Messiah’s resurrection.
John's baptism was, and is, a valuable sign, indeed as the apostle points out for testifying of the Messiahship of Yeshua.
There is no evidence for a new water baptism in 1 John, only a powerful testimony of the continued importance to the early disciples of the end-time Messianic purification baptism announced to all in Israel by R. John.