26 C.F.R. § 1.871-3
In order to determine whether an alien seaman is a resident of the United States for purposes of the income tax, it is necessary to decide whether the presumption of nonresidence (as prescribed by paragraph (b) of § 1.871-4 ) is overcome by facts showing that he has established a residence in the United States. Residence may be established on a vessel regularly engaged in coastwise trade, but the mere fact that a sailor makes his home on a vessel which is flying the United States flag and is engaged in foreign trade is not sufficient to establish residence in the United States, even though the vessel, while carrying on foreign trade, touches at American ports. An alien seaman may acquire an actual residence in the United States within the rules laid down in § 1.871-4 , although the nature of his calling requires him to be absent for a long period from the place where his residence is established. An alien seaman may acquire such a residence at a sailors' boarding house or hotel, but such a claim should be carefully scrutinized in order to make sure that such residence is bona fide. The filing of Form 1078 or taking out first citizenship papers is proof of residence in the United States from the time the form is filed or the papers taken out, unless rebutted by other evidence showing an intention to be a transient.
26 C.F.R. §1.871-3