Captain's Maneuver — ruling 1

If the second target is no longer on the battlefield, or if the second target is no longer a creature, player, or planeswalker when the damage would be redirected, the damage is dealt to the first target as if this spell was not cast.

Captain's Maneuver — ruling 2

If the first target is no longer on the battlefield, then the damage will not be dealt at all, so it can’t be redirected by this ability.

Captain's Maneuver : règles, fonctionnement et interactions (MTG)

Captain's Maneuver is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

The next X damage that would be dealt to target creature, planeswalker, or player this turn is dealt to another target creature, planeswalker, or player instead.

How it works : Captain's Maneuver

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
If the second target is no longer on the battlefield, or if the second target is no longer a creature, player, or planeswalker when the damage would be redirected, the damage is dealt to the first target as if this spell was not cast.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
If the first target is no longer on the battlefield, then the damage will not be dealt at all, so it can’t be redirected by this ability.
No other card is named in this ruling.