The opportunity chain was finished before the problem occurred. She only went home with the chain unfinished once in all the times I played through trying to find the problem. Then she completed it as soon as there were debris piles to excavate in the changed future. And with both the sim I was playing with when I originally started this thread and the current sim, I waited for two or three sim days with none of the floating objects ever appearing on the water. Maybe I should try it sometime without doing any of the opportunity chains.