Kenesha Antoine and boyfriend Steven Weber were on what Antoine described as her "bucket list" vacation.
Video shows Weber swimming up to the window Thursday afternoon of the underwater Manta Resort, a partially submerged structure anchored to the seafloor that gives guests a look into the ocean.
Weber then shows Antoine, who is inside the room, a piece of paper with a note asking her to marry him.
The note read: "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! Will you please be my wife? Marry me???"
Weber, who wore a snorkel and flippers, gives Antoine time to read the two-sided note before reaching into his pocket to pull out a ring. He then swims out of frame.
The Manta Resort told the BBC that Weber, "tragically drowned while free diving alone outside the underwater room."
After Weber's death, Antoine posted a message to him on Facebook with what she would have told him.
"You never emerged from those depths, so you never got to hear my answer, "Yes! Yes! A million times, yes, I will marry you!!" 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 cruelest twist of fate imaginable," she wrote.
The Department of State issued a statement about the incident:
"We are aware of reports of the death of a US citizen in Tanzania. We offer our sincerest condolences to the family on their loss. We stand ready to provide all appropriate consular assistance. Out of respect to the family during this difficult time, we have no further comment."