This buff is retained even if you abandon the quest, after which you can talk to Exarch Admetius to get it again. However, it only works around Blood Watch and will automatically dispel if you leave the area.
It also causes you to speak Thalassian no matter what language is selected in the interface. No Alliance players can understand this language, even those under the effect of this disguise (including yourself).
It does not always preserve your gender.