In Event of Moon Disaster arose via the suggestion of Bill Anders (an astronaut on Apollo 8) who let the White House know they should be prepared in case of lunar disaster. Frank Borman, the NASA liason at the White House, informed the speechwriter Bill Safire that "you might consider an 'alternate posture'" for Nixon. Safire didn't understand until Borman added: "like what to do for the widows."
To be clear, this was a speech intended not for all three crew members, but for Neil Armstrong and Edwin Aldrin; the lunar landing and recovery being dangerous enough that it was consider quite possible both astronauts would be lost; Michael Collins, who did not go on the lunar craft, was expected to make it home and was not included in the speech.
The widows-to-be were meant to be informed first, and then Nixon's speech was to be given before NASA ended communication with the men. Here is the first part.
Fate has ordained that the men who went to the moon to explore in peace will stay on the moon to rest in peace.
These brave men, Neil Armstrong and Edwin Aldrin, know that there is no hope for their recovery. But they also know that there is hope for mankind in their sacrifice.
Here is the end:
For every human being who looks up at the moon in the nights to come will know that there is some corner of another world that is forever mankind.
After the speech, communication was then to be broken; after, and only after, was the last part meant to happen -- a burial at sea by a clergyman, commending the souls of the astronauts to the "deepest of the deep."
Since the speech didn't have to be used, let's end on a more positive note: Safire also worked on the plaque on the Apollo 11, derived from NASA's own declaration of purpose.
HERE MEN FROM THE PLANET EARTH
FIRST SET FOOT UPON THE MOON
JULY 1969, A.D.
WE CAME IN PEACE FOR ALL MANKIND
The plaque was incidentally meant to read "first landed" but because of concerns the Soviet Union might get a robot to the moon first (they didn't until 1970), this became "set foot".
It was Safire who changed the original "we come in peace" to "we came in peace".
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Safire gave a 1999 interview on Meet the Press about the speech.
Other sources:
Launius, Roger. Reaching for the Moon: A Short History of the Space Race. Yale University Press, 2019.
Logsdon, John M. "Richard Nixon and Apollo 11." After Apollo?. Palgrave Macmillan, New York, 2015.