Just to be pedantic CY, "African" is not synonomous with negro. The evidence both acedemic and empirical would indicate that the seafaring tradition in Africa started and was concentrated around the Mediterranean and Red Sea coasts, not those areas below the Sahara. I agree that the use of "call - response" work songs is widespread in the African continent, but I would argue that this suggests a parallel development to a common situation, rather than radiation from an original source. Similarly the use of shanties or shanty form chants by Polynesians has already been mentioned, so I would very much doubt your hypothesis, even without the evidence of brother Fabri.
Having said that I must agree about the influence of seamen from the West coast of Africa on the development of those shanties still extant.
Following on from Barry's comments about them being 'the cream of the crop' there is a documented incident of a ship hiring crew in Glasgow (in the 1880's I believe) whose master displayed a prominent sign that he was hiring crew but that no Irish or Niggers need apply. Of course members of both these communities came to remonstrate with him and he "allowed" himself to be persuaded to "make an exception" for the best those who showed up. He ended up with one watch of Irish and one of negros between which he encouraged rivalry and achieved one of his fastest and most efficient trips.