We have lingered in the chambers of the sea By sea-girls wreathed with seaweed red and brown Till human voices wake us, and we drown.
This is kind of an "it was all a dream" ending, but even weirder.
Prufrock brings "us" back into the picture, saying that we have been hanging out in the ocean with him.
The word "chambers" has two meaning here: it can refer to small cramped spaces, or it can refer to rooms, especially bedrooms.
Remember that Prufrock has spent significant amounts of time lurking outside of rooms and imagining women who are wrapped in shawls and laying on pillows. We don’t know who the "sea-girls" are, but they don’t seem quite as majestic as the mermaids.
The "human voices" may remind us of the "voices with a dying fall" from line 52.
Oh, and by the way, we’re dead. We drowned with Prufrock. No!
Seriously, though, we can’t help you much with this ending. It could signal that Prufrock has truly grown insane, or that his "true self" is really more crab-like that human, or that, yes, he has been dreaming the whole time. (We don’t really buy the dream story, but if that’s your thing, go for it.)
One thing is clear: Prufrock’s story does not turn out well. It does not turn out well, at all.