Do you have an obligation to go to the mikvah erev Yom Kippur.
Rambam writes in his preface to his Commentary on the Mishnah that one who rejects "minhagei Yisrael" - customs that are considered valid and universal in Torah Judaism, on him the verse is applied "One who knocks down a fence will be bit by a snake." (Definitely something bad!)
That being said, the custom to dip in the mikvah on erev Yom HaKippurim is brought in Shulhan Aruch, O.C. Siman 606, S'if 4. It is universally accepted among all Torah Observant Halachic authorities, Ashkenazic, Sefardic, Yemenite, etc. Even Rambam is referred to as one of its sources! The Aruch HaShulhan uses the wording "Nahagu kohl Yisrael (All Jews are accustomed...). (Ibid. Si'if 5) It definitely fits the term "minhag Yisrael"!
It is referred to as a mitzvah, not as hovah, literally meaning that it is not obligatory. (Kitzur SA 131,6) This just means is that the precautions taken for an immersion that is actually required by the Torah do not apply. It does not mean that the minhag is optional. If one absolutely cannot immerse (e.g. too sick), then the pouring of 9 kabin of water over the body is acceptable for this immersion. There are those who maintain that standing completely under a fully opened shower for a while would fulfill the 9 kabin requirement. (Halichos Olam Page 179)