He tied it to a sand dollar, and called her attention to it.
When she grabbed it, the ring fell and then vanished in between the rocks.
The frantic couple asked beachgoers and lifeguards to help in the search.
"Every single rock around where it fell was moved, and there was a flashlight and ropes involved," said Cawley's girlfriend. "My heart broke for Matt because I know he put so much effort."
Another ring has been ordered, and the couple also learned Cawley's mother had the family heirloom insured, just in case.
