As Choked Estuary enters the battlefield, you may reveal an Island or Swamp card from your hand. If you don't, Choked Estuary enters the battlefield tapped.
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4/8/2016 You may reveal any land card with either or both of the appropriate subtypes. It doesn’t have to be a basic land. For example, you could reveal Prairie Stream from the Battle for Zendikar set to satisfy the ability of Choked Estuary.
4/8/2016 Lands don’t have a subtype just because they can produce mana of the corresponding color. Choked Estuary itself is neither an Island nor a Swamp, even though it produces blue and black mana, so you can’t reveal one to satisfy the ability of another.
4/8/2016 If an Island or Swamp is entering the battlefield from your hand at the same time as Choked Estuary, you may reveal the other land to have Choked Estuary enter untapped.
4/8/2016 If an effect instructs you to put Choked Estuary onto the battlefield tapped, it will still enter the battlefield tapped even if you reveal a land card from your hand.