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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.