Steven Weber and his girlfriend, Kenesha Antoine, were staying in a submerged cabin at the Manta Resort, off Pemba Island.
Footage shows Mr Weber diving under water to ask Ms Antoine to marry him.
In the video, Mr Weber presses a hand-written proposal note against the cabin window as Ms Antoine films from inside.
Ms Antoine, confirming Mr Weber’s death in a Facebook post, said he “never emerged from those depths”.
The Manta Resort told the BBC Mr Weber “tragically drowned while free diving alone outside the underwater room” on Thursday afternoon.
“It is with the deepest regret that we inform that a fatal accident occurred at The Manta Resort on Thursday 19 September, 2019,” the resort said in a statement.