Through the entire movie, Rob had the whistle around his neck, but he lost everything in the raft except his toothbrush. However, there are various pieces that continue to float around him, and it's not until after he has ransacked the sub that the whistle actually makes its first appearance, making it more likely he got the whistle from the sub.
When Tanamashu is "talking" to Kabuna, he is speaking in English, a language which he wouldn't speak to his God with. However, this could easily be explained as Wednesday, herself, could speak English "fluently," as well as all the other tribal women doing the same. It's also a fact that Tanamashu had heard other dialects of English since he did recognize that Kaboona spoke, out loud, with a "white-man dialect." Also the fact that he had an umbrella over his head almost constantly, and that he recognized a torpedo makes it apparent that, despite the relative uniqueness of the tribe, they have had considerable contact with the rest of the world, making the English talking to Kaboona more understandable.