In The Goblet of Fire before going into the lake no one told Harry what to do. It actually was the whole point that the champions should have guessed themselves about saving someone they love. Bagman didn't specify anything for them before the beginning of the second task:
“Well, all our champions are ready for the second task, which will start on my whistle. They have precisely an hour to recover what has been taken from them. On the count of three, then. One ... two ... three!”
Dobby told Harry that it was Ron they took for Harry to save. But Dobby was not official and it was kinda cheating. So how was this planned by the organizers? They surely knew that both Ron and Hermione were Harry's best friends and there was a possibility they knew about his feelings for Cho. Let's say he is coming down and sees all the "prisoners" - how on Earth he should know whom he should save, given he is pressed on time to do the calculations about other champions? What if he would have chosen Hermione and left? Or worse - Cho to impress her?