The activation of only the card "Sealing Ceremony of Suiton" can be done at any time, except during the Damage Step.[1]
The effect "Once per turn: You can send 1 WATER monster from your hand to the Graveyard to target 1 card in your opponent's Graveyard; banish that target" is an effect that makes a Chain Link. (It targets 1 card in your opponent's Graveyard. It cannot be activated during the Damage Step.)[1]
When activating the effect "Once per turn: You can send 1 WATER monster from your hand to the Graveyard to target 1 card in your opponent's Graveyard; banish that target", send 1 WATER monster from your hand to the Graveyard as the cost.[1]
The card "Sealing Ceremony of Suiton" and the effect "Once per turn: You can send 1 WATER monster from your hand to the Graveyard to target 1 card in your opponent's Graveyard; banish that target" can be activated in the same Chain Link. (However, neither the card "Sealing Ceremony of Suiton" nor its effect can be activated during the Damage Step.)[1]