Our discussion was two fold. Who gets to direct the damage (pick a new target) and let's say it was cast before combat damage was assigned. Who determines which damage gets redirected. The active player or the owner of the spell. My assumption is the person who cast the spell determines which damage and where to redirect it.
Captain's Maneuver: The next X damage that would be dealt to target creature or player this turn is dealt to another target creature or player instead.
The controller of the spell (who is the person that cast it) gets to decide which creature's damage is replaced, and where that damage goes.
When you cast a spell, you choose the targets for that spell. In the case of Captain's Maneuver, there are 2 targets that you choose. One is the creature that would receive damage, and the other is the creature that receives the damage instead. This works the same with Captain's Maneuver as it would with any other targeted spell such as Lightning Bolt.
- Casting Spells
601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect.
601.2c The player announces his or her choice of an appropriate player, object, or zone for each target the spell requires.
