Ethan Studenic, 30, popped the question to Morgan Whittaker, 28, with an engagement ring concealed inside a shell.
The lab administrator pretended to pick something up from the ocean floor but instead produced a ring and asked his girlfriend of 10 years to marry him.
Morgan, a social worker, immediately nodded yes and the couple embraced before swimming up to the surface together.
Ethan, from Roanoke, Virginia, USA, had been planning the underwater proposal for five years.
He said: ‘Five years ago, I had the idea of an underwater proposal.
‘I thought it would be cool, really unique and interesting.
‘I decided to propose on the first reef we dove together.’
He took Morgan to the Caribbean island of Bonaire, where they had gone on their first vacation.
‘It was a really special trip for us because Morgan had never been out of the country before or flown on a plane,’ he said.
‘She had never been scuba diving before.
‘I had been scuba diving for several years and I really wanted to share it with her.
‘We went and had a great time and made a lot of special memories.
‘I wanted it to be a romantic proposal and I’m not the greatest at that kind of thing.
The piece of jewellery featured crushed sea glass that Ethan found on a beach on the island.
‘There is a bunch of glass that washes up on the beach off that reef,’ Ethan explained.
‘We had taken a couple of pieces home the first time we went.
‘I had it crushed up and embedded in the engagement ring.’
Ethan hid the wood and glass ring inside a case made of shells.
The engagement happened in July 2019 and they plan to tie the knot in April this year in front of 100 of their friends and family.