Uber driver Seth Dixon got down on one knee to pop the question to his girlfriend Ruth Sales at their favourite spot in Kansas City's Loose Park.
But footage captured by a pal showed the ring fly out of the box before slipping through the tiny cracks of a bridge and into the water below.
Dating for four years, Ruth can be heard telling Seth: "It went down."
Not willing to let the ring go without a fight, the couple's friends grabbed their goggles and dived in to search for it.
They have also put together a GoFundMe page to raise money to replace the ring .
According to the fundraising page "after a long, hard search all the divers came up empty handed. The couple left with no ring in hand but a heart full of love and memories to last a lifetime".
It added: "This couple might not have the picture perfect engagement story but they do have a story they hope will bring joy and laughter to you as you read it. Sometimes our love stories aren't always as perfect as we planned, but always work out in the end!"
Ruth and Seth are due to get married next month on October 21.
Seth told the Kansas City Star that the whole thing is 'absolutely real'.
"I wish it wasn't", he added. Ruth, who is a substitute teacher, also said that the pair both 'freaked out'.
"It ping-ponged from one plank to another. You could hear a little plop," she said.
Ruth said despite what happened she's just 'happy and excited that we're finally engaged'.
"It's been a roller coaster of emotions to be honest," she said. "But I know God will work everything out."