Tevilah with dreadlocks is acceptable.
The Rema cited by @Yehoshua is coming from the Beis Yosef ad loc., who is quoting the Mordechai in Shavuos §751, who is quoting a Ra’avyah in Teshuva 991. The Ra’avyah gives three reasons why these plaits (see here for background) are not a problem of chatzitza:
- The halacha (see Niddah 67a, Shulchan Aruch YD 198:5) is that one hair tied in a knot is tight enough to be considered a chatzitza. More than one hair is not. This is more than one hair.
- Even if it is tight; since whatever it naturally covers is
considered “swallowed up” regarding technical laws of tum’ah, one
could posit that this also has bearing on the laws of tevilah in
that it is not necessary for that area to undergo tevilah.
- Since the assumed danger involved in the removal of these locks will
cause this woman to want them to remain there, we say “that is the
way it grows”; a line of reasoning which says that anything which is
intended to be there (not just that one doesn’t mind it’s presence)
is considered part of the body itself regarding tevilah.
There is a fourth reason which is really much more basic: Anything which a person does not intend to remove is not a chatzitza unless it covers most of the body, and even then it is only a chatzitza mid’rabannan (see Niddah ibid.). One who examines the words of the Ra’avyah inside will notice that this is a sufficient reason which he only ignored because of the opinion that most of the head is just as bad as most of the body, rendering this point moot.
At any rate, we have at least three reasons why the braids are not a chatzitza, and all of them apply to dreadlocks. Even though the third reason – that “that’s the way it grows” – was clearly said because of the assumed danger in removing it, we find the exact concept applied when something is done for beauty; namely further on in the Shulchan Aruch (s’if 17) where hair dye is not considered a chatzitza for this very reason.
There is a Rema in the first se'if which says that the custom is to remove all interfering items; even ones which would not be considered chatzitzos. However, see the Sidrei Tahara there who writes that this is only an extra stringency, and the Aruch Hashulchan similarly writes that if there is any need whatsoever one should not be stringent about this. So although it is a laudable custom, one should certainly be aware of the letter-of-the-law.