Did I walk out of the room during the part where Downey figures out how his mom actually died? I Heard the psychiatrist raising a question about Downey's memory that his mom drowned.
It's been a while since I've seen the movie, but in the miniseries, they intentionally leave it very ambiguous as to how his mother died, so it could well be the same in the movie.
I seem to recall that in the mini-series the death of Marlow's mother was explicitly stated - She committed suicide by jumping off Hammersmith Bridge (?) into the Thames - Marlow himself states this when Dr. Gibbon asks him how his mother died.