I think it all depends on the context. Grammatically it�s possible. �Could� would refer to a possibility (tentative future in the past), so we would have three time references: the time focused in the story (past: they were in the water),  a possibility of something happening in the future of that past (being killed) and a past in the past (they had not seen animals before they jumped in) If it sounds strange is only because we�re in the middle of the story, there�s a lack of context but to me it�s perfectly OK.