The source for not showering after immersing in the mikvah is Mesechta Shabbos Daf Yud Daled Amud Alef:
http://www.hebrewbooks.org/shas.aspx?mesechta=2&daf=14&format=pdf
Translated in english:
And one who comes with his head and the greater part of his body] into drawn water, why did the Rabbis decree uncleanness in his case? — Said R. Bibi in R. Assi’s name: Because originally people performed tebillah in collected pit water, which was stagnant [noisome], and so they poured drawn water upon them selves. [But when] they began to make this a fixed [law], the Rabbis imposed uncleanness thereon. What is meant by ‘a fixed [law]?’Abaye said: They maintained, Not this [pit water] purifies, but both together purify. Said Raba to him, Then what did it matter, seeing that they did perform tebillah in this [the pit water]? But, said Raba, they maintained, Not this [the pit water] purifies but that [the drawn water].