"eating fish alive" does indeed sound unnatural, both semantically and pragmatically!
Nevertheless, "boiling them alive" sounds fine, and brings us back to the original structure again.
How about thinking of phrasal verbs like "throw away" and "take apart" as a means of analyzing the situation before us?
"The child threw the toy away." The toy is 'away' as a result of
the operation. But is it therefore an 'away toy' we are talking
about? At the end of the event it is, perhaps, a discarded toy, but
not an away toy.
"The child took the toy apart." The toy is 'apart' as a result of
the operation. But is it therefore an 'apart toy' we are talking
about? At the end of the event it is, perhaps, a disassembled toy, but
not an apart toy.
The child can throw the toy or throw it away. The child can take the toy, or take it apart.
I see these as four separate verbs: "to throw", "to throw away", "to take", and "to take apart".
Similarly, although "alive" is not the resultant state,
"They burned the woman alive." The woman is 'alive' during the
operation. But is it therefore an 'alive woman' we are talking
about? During the event she is, perhaps, a living woman, but not
an alive woman.
I think we are concerned here more with the manner of doing the burning
than with the state of the woman, strange as that may seem. We
are more interested in describing the burning than the woman.
That it is a living woman is part of the definition of "woman" already,
so to say that "alive" describes the woman is to say that "alive" is
placed in the sentence redundantly. We say, "The woman bought a
dress", not "The living woman bought a dress."
I propose that 'alive' is an adverbial particle used to form phrasal
verbs just the same as shown in the examples "throw away" and "take
apart".
Thus, we have the phrasal verbs "burn alive", "boil alive", and "bury
alive", each denoting a particular form of execution. In this
analysis, "alive" cannot possibly be an adjective - unless we are also
willing to accept "away" and "apart" as adjectives. Beside these verbs
we have "burn", "boil", and "bury". "burn" is just as different
from "burn alive" as "take" is from "take apart".
CJ