He confirmed to Fancast that the instalment would see the return of Aquaman (Alan Ritchson) and would also introduce the character's wife Mera (Elena Satine).
"We're going to shoot underwater, with dolphins [and] with explosions," he explained. "It's going to be a huge episode."
The episode, titled 'Patriot', will mark Welling's sixth stint as a director on the show. He made his directorial debut with the fifth season episode, 'Fragile'.
The final season of Smallville continues on Fridays at 8/7c on The CW.