Does "the placed card" in this card's ＜Main Timing＞ effect refer to the Digimon card with [Aqua] or [Sea Animal] in one of its traits placed from my hand as the bottom digivolution card, or does it refer to my blue Digimon?

It refers to the Digimon card placed from the hand as the bottom digivolution card.

Sea of Destruction rulings (EX6-066)

1 official ruling from Bandai, last ruled Jun 28, 2024. Straight answers to how this card actually resolves.

Effect: [Main] By placing 1 Digimon card with [Aqua] or [Sea Animal] in any of its traits from your hand as the bottom digivolution card of 1 of your blue Digimon, return all of your opponent's Digimon with the same level as the placed card to the hand.
Security effect: [Security] Return all of your opponent's Digimon with the lowest level to the hand.

Official Q&A

Does "the placed card" in this card's ＜Main Timing＞ effect refer to the Digimon card with [Aqua] or [Sea Animal] in one of its traits placed from my hand as the bottom digivolution card, or does it refer to my blue Digimon?
It refers to the Digimon card placed from the hand as the bottom digivolution card.
Ruled Jun 28, 2024

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Questions and answers are Bandai's official rulings, collected and translated by the DigimonCardGame Wiki community and reused here under CC BY-SA. Card names and card text are © their respective publishers; Combalo is not affiliated with Bandai.

Sea of Destruction rulings (EX6-066)

1 official ruling from Bandai, last ruled Jun 28, 2024. Straight answers to how this card actually resolves.

Effect: [Main] By placing 1 Digimon card with [Aqua] or [Sea Animal] in any of its traits from your hand as the bottom digivolution card of 1 of your blue Digimon, return all of your opponent's Digimon with the same level as the placed card to the hand.
Security effect: [Security] Return all of your opponent's Digimon with the lowest level to the hand.

Official Q&A

Does "the placed card" in this card's ＜Main Timing＞ effect refer to the Digimon card with [Aqua] or [Sea Animal] in one of its traits placed from my hand as the bottom digivolution card, or does it refer to my blue Digimon?
It refers to the Digimon card placed from the hand as the bottom digivolution card.
Ruled Jun 28, 2024

Got a line that uses Sea of Destruction? Map it in the builder and share it with a link.