What is the test for maintenance rights in maritime law?

Vaughan v. Atkinson, 369 U.S. at 531, 82 S.Ct. 997 (internal quotation marks omitted). As a right judicially fashioned to protect "poor, friendless and improvident" seamen,

[m]aintenance ... differs from rights normally classified as contractual. As Mr. Justice Cardozo said in Cortes v. Baltimore Insular Line, supra, [287 U.S. 367,] 371, 53 S.Ct. 173, 77 L.Ed. 368 [1932], the duty to provide maintenance... "is imposed by the law itself as one annexed to the employment.... Contractual it is in the sense that it has its source in a relation which is contractual in origin, but given the relation, no agreement is competent to abrogate the incident."