It's a sailboat in the background
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Archived from the IMDb Discussion Forums — The Boy in the Plastic Bubble
slackline70 — 10 years ago(February 26, 2016 02:07 AM)
That 'ghostly figure' in the background at the end when they're kissing?
It's the sail of a sailboat. In fact you can see another sail further off to the left.
Once you realize this, it doesn't even look that creepy. It's only creepy if you believe you're seeing a person, since it glides along instead of displaying a typical ambulatory motion.