Rulings

You announce how the damage will be divided as part of putting Orca, Siege Demon's last triggered ability on the stack. Each chosen target must receive at least 1 damage.
If Orca, Siege Demon's last ability has multiple targets, and some but not all of them are illegal targets when the ability resolves, Orca, Siege Demon will still deal damage to the remaining legal targets according to the original damage division.
If all of the targets are illegal when the ability tries to resolve, it won't resolve and none of its effects will happen. Nothing will be dealt damage.