1. Someone Shinichi and Ran met at a beach when they were kids. This would explain a) why Ran remembers seeing her back (they were playing) and the sound of waves and b) why Sera recognized Shinichi immediately. She probably looked and saw him in the news back in America, and recognized him, making that one of the reasons she left to Japan (the other being because of her brother).
2. -Someone else posted this theory on youtube, I belive-
Someone Shinichi and Ran met when they went to New York. I think that trip took place when they were in Grade 8, so the timing does fit.
I'm personally going with my theory because I think that it would make sense as to how she recognized him that fast if she'd seen him before when they were kids (Ran's kind of...well I don't want to call her dumb, but she has been duped into thinking he's not Shinichi, even when recognizing him to be so). Maybe I'm just biased though XD ?
Anyways, vote and tell me what you think!