During the process of immersion water must be able to reach all parts of the body at the same time, including each strand of hair. If the hair is worn in locs or is braided, this is not possible. Therefore, it is normally prohibited to immerse in this manner.
A woman with locs who is planning to get married, or to begin observing the laws of mikveh, should approach her rabbi, mentor, or kallah teacher for individual guidance.