It was meant to be an incredibly romantic gesture but it ended up in tragedy.

Steven Weber Jr, from the US, drowned after an underwater proposal to his girlfriend Kenesha Antoine during the couple's trip to Tanzania this week.

The grieving girlfriend took to Facebook to confirm the death of her fiance, posting a touching tribute sharing their final moments together.

"There are no words adequate enough to honour the beautiful soul that is Steven Weber, Jr.," she wrote.

"You were kind, compassionate, you regularly made me cry with laughter, and you showered me with a love like none I'd ever experienced."

She shared a video of Weber, showing him in diving gear with a note sealed in a plastic bag.

"I can't hold my breath long enough to tell you everything I love about you. But... Everything I love about you I love more every day," the note read.

Weber flipped the paper over: "Will you please be my wife. Marry me???"

"Yes! Yes! A million times, yes, I will marry you!!" she responded. But he never returned to the surface to hear those words.

"We never got to embrace and celebrate the beginning of the rest of our lives together, as the best day of our lives turned into the worst, in the cruellest twist of fate imaginable."

"I will find you and marry you in the next lifetime, and the next, and the next, and the next... I love you so much, and I always will."

It is not yet known of caused Weber's death.