Gibb, I'm guessing the research budget was comparatively small. Any expert advice presumably went to more obvious and interesting questions, like what a real pirate ship looked like, and whether pirates carried automatic weapons.

The simplest explanation for the singing style is probably that the music director was so used to English- or Irish-accented shanties that it never occurred to him or her to vary it. Or if it did, the task might have been too daunting for the time and talent available and the wages offered. (If you're supplied with a bunch of white shanty singers, for example, it could be a PR disaster to urge them to sing more like what you or they think 19th C. Africans should have sounded like. And there may have been no time or money to hire additional black "shantymen.")

It sucks when people don't want to draw on one's knowledge or, worse, acknowledge one's efforts. But video-game people on deadlines aren't likely to cover all bases. And in the business world, unfortunately, acknowledgements are usually given only if legally required.

If pop culture dropped the ball (again), maybe it can retrieve it later.