OOC: secondary source: what is Moahu?
From the indispensable reference Harbors and High Seas:
"[Aubrey and Maturin are] told to resolve a power struggle on the fictional British island of Moahu in favor of whichever side he perceives is most likely to acknowledge the king's sovereignty ...
"O'Brian informs us at various times that the volcanic island of Moahu is 'to the south of the Sandwich group' and 'no great way from Hawaii'. In fact, the name 'Moahu' seems to be inspired by a phonetic combination of 'Mowee' and 'Woahoo,' early renditions of 'Maui' and 'Oahu', respectively. We also know that Moahu belongs to the British, 'Captain Cook having taken possession of the archipelago in 1779,' the same year Cook was murdered at Kealakua Bay, Hawaii. The Surprise approaches Christmas Island, which was discovered by Cook in 1777, from the south. At about a three days' sail from Moahu it appears on the eastern horizon to starboard. Moahu -- where the novel's action reaches its climax -- would lie then at about 10°N, somewhere between the Line Islands and the Sandwich (Hawaiian) Islands."
-- Dean King, Harbors and High Seas: An Atlas and Geographical Guide to the Complete Aubrey-Maturin Novels of Patrick O'Brian, pp. 176-180
"[Aubrey and Maturin are] told to resolve a power struggle on the fictional British island of Moahu in favor of whichever side he perceives is most likely to acknowledge the king's sovereignty ...
"O'Brian informs us at various times that the volcanic island of Moahu is 'to the south of the Sandwich group' and 'no great way from Hawaii'. In fact, the name 'Moahu' seems to be inspired by a phonetic combination of 'Mowee' and 'Woahoo,' early renditions of 'Maui' and 'Oahu', respectively. We also know that Moahu belongs to the British, 'Captain Cook having taken possession of the archipelago in 1779,' the same year Cook was murdered at Kealakua Bay, Hawaii. The Surprise approaches Christmas Island, which was discovered by Cook in 1777, from the south. At about a three days' sail from Moahu it appears on the eastern horizon to starboard. Moahu -- where the novel's action reaches its climax -- would lie then at about 10°N, somewhere between the Line Islands and the Sandwich (Hawaiian) Islands."
-- Dean King, Harbors and High Seas: An Atlas and Geographical Guide to the Complete Aubrey-Maturin Novels of Patrick O'Brian, pp. 176-180