Subbing a ball on the green
You used a beat-up ball to carry the water on a par 3, and you hit the green. If you switch to another ball to putt, you’d violate Rule 15-2 (resulting in loss of hole in a match or a two-shot penalty in stroke play). You can sub a ball during the play of a hole only if a rule permits you to do so (the original is deep in a water hazard), or if it has become unfit during play on that hole (e.g., cut, cracked or out of shape). Being scuffed doesn’t count. If you think it’s unfit, notify your group about your intention to replace the ball so they’re given a chance to inspect the ball before you swap it.