What I understand is this: Baptism did not originate with John... John the Baptist was already baptizing in the Jordan River before Jesus came to him and many Jews came to him to be baptized. They understood the ordinance and its necessity. John was not teaching some new doctrine that the Jews did not understand. They came to John because they recognized him as a prophet with authority to baptize.
All these things came to and through the House of Israel because it was to them that the original prophesies were given. Remember, "baptism" is a Greek word, not a Hebrew one. To the Jews, before New Testament times, it was simply referred to as the ordinance of "washing". Here are some references that I believe refer to these "washings":
Isaiah 48:1: "Hear ye this, O house of Jacob, which are called by the name of Israel, and are come forth out of the waters of Judah, which swear by the name of the Lord, and make mention of the God of Israel..."
The waters of Judah mentioned here has reference to the fact that the Jews knew that the Messiah was to come through the tribe of Judah, but rather than use the phrase, "waters of the Messiah" (in who's name they must be washed) waters of Judah was used instead to protect the sacred name. Notice as well, Isaiah says "and". One has to be called by the name of Israel "and" be baptized (to use the Greek term). I happen to believe that I am adopted into the House of Israel. Specifically into the tribe of Ephraim the son of Joseph. I believe this is what the symbology of Joseph's coat of many colors had reference to; that all who are washed in the name of the Messiah would be counted as sons of Abraham (through Joseph) to whom it was said that through him would "all" the kindreds of the earth be blessed. Joseph's coat of many colors = all races.
In I Kings 7:23-26 we have this:
23 ¶And he made a molten sea, ten cubits from the one brim to the other: it was round all about, and his height was five cubits: and a line of thirty cubits did compass it round about.
24 And under the brim of it round about there were knops compassing it, ten in a cubit, compassing the sea round about: the knops were cast in two rows, when it was cast.
25 It stood upon twelve oxen, three looking toward the north, and three looking toward the west, and three looking toward the south, and three looking toward the east: and the sea was set above upon them, and all their hinder parts were inward.
26 And it was an hand breadth thick, and the brim thereof was wrought like the brim of a cup, with flowers of lilies: it contained two thousand baths.
Solomon was not doing anything but what the Lord or the law commanded him to do. Do you suppose this vessel (molten out of brass) had any other purpose than the ordinance of washing? Consider the two thousand baths. Would not that volume of water suggest immersion.
The references are scarce and maybe a bit vague, but I believe they are there. Besides, being LDS, I believe revelation given through Joseph Smith clarifies this issue beyond doubt.