Complex sentence: However, whether or not the island that he saw really was Tahiti or not has not been proved with any certainty, and it has been suggested that he actually saw the island of Rekareka to the south-east of Tahiti.
Split into simpler sentences: However, whether the island that he saw was actually Tahiti or not has not been fully ascertained. It has been suggested that he actually saw the island of Rekareka to the south-east of Tahiti.