Question

The water was then carried home for invalids (change the voice)

Answer 1

If it is:
The water was then carried home for invalids (by them)

Then:
Then they carried water home for invalids.

Answer 2

Let me clear you first, As mention there, we have to change the voice but this is not mentioned that active to passive, so this is a little confusing question, because we have to change passive to active, so the answer is Carried the water home then for invalids