One explanation is that she heard the name of the island pronounced that way by her father's crew in the 1890s.

Another is that when R. M. Davids gave her "The Lass of Mohee," he told her it meant Maui.

Or someone else told her the pronunciation had changed, and she added a footnote to indicate how it was pronounced in the old days.

William Nevens's "Forty Years at Sea" (1845) also spells it "Mohee."

Mohea, or Pulo Mohea, is a small island off the west coast of Thailand. (Conceivably it is the locale, chosen at random, of "The Lass of Mohea." That might explain why she's an "Indian" lass.)

If, as seems likely, Hugill colloquializied some of the words and even created the final stanza, why wouldn't he also have "corrected" the pronunciation to "Maui"?