The Nazi Gestapo referred to Jews in hiding as “U-Boats” to survive under the Third Reich. Like the seagoing vessels, they traveled a fluid sea. Some navigated below the depths, while others floated on the surface in plain view, artfully (and not so artfully) disguised to conceal their actual identities. The article profiles the author’s family, the Mahlers of Antwerp, Belgium, during the Nazi occupation through an investigation of the historical, theoretical, and discursive categories of “race” and “race passing,” specifically as it relates to European-heritage Jewish people.