Wavecrash Triton's ability can target a creature that's already tapped. That creature still won't untap during its controller's next untap step.

Heroic abilities will resolve before the spell that caused them to trigger.

Heroic abilities will trigger only once per spell, even if that spell targets the creature with the heroic ability multiple times.

Wavecrash Triton's ability tracks the creature, but not its controller. If the creature changes controllers before its first controller's next untap step has come around, then it won't untap during its new controller's next untap step.

Heroic abilities won't trigger when a copy of a spell is created on the stack or when a spell's targets are changed to include a creature with a heroic ability.