As in most of the answers I believe it all comes down to culture. I think a good analogue of this is within the Frank Herbert's Dune Universe.
On Dune (A mostly barren desert planet with very limited water resources) the native inhabitants Freemen recycle and reuse all water including capturing exhaled breath, bodily waste and even the processing of corpses. Such is the value of water, that spitting is considered a very high form of respect (An activity usually frowned upon in other cultures). I believe the key sentiment in the processing corpeses is that the dead are dead and require their water (or other consitutants) less than the living.
To Quote:
“We will treat your comrade with the same reverence we treat our own,” the Fremen said. “This is the bond of water. We know the rites. A man’s flesh is his own; the water belongs to the tribe.”
The other half of the solution is that in a ship that travels for generations with a small group of people with presumably little outside contact it becomes much easier for social norms to be transformed over time.