We don't know for a fact that Robin went or fell in the water. While I agree that for Rachel, it's a natural thought process to not think it happened that way, it could of happened just that way. Unfortunately, until we know the results of the autopsy, we don't have an answer as to what exactly caused Robin's death.
Maybe, and it's just speculation on my part, Rachael has a hard time believing her mother went or fell in the water because of the many years her mother had lived there, nothing of that nature ever had occurred in the past. Couple that with the fact that the dog died as well, and that the neighbor said on video something to the effect of Robin and Bella did not routinely play on the pier, and the mention earlier in this thread that Bella was deathly afraid of the water. Rachel would know the habits of her mother and Bella.
I also doubt Rachael has been told much as to not jeopardize the investigation but she has a right to her own speculation about what may have occurred that night. That's also not to imply that she thinks her father is culpable, for all we know she could suspect someone else. There was a another woman and a dog that were killed in that area shortly after Robin went missing. In addition, Rachel being close to the situation, she would also know the habits and more recent behavior of her father which may or may not be unusual. When thinking about what Rachel said to the reporter, we have to consider all of this and more because Rachael is inside the situation and obviously has more insight than any of us would.
