Disrupting Shoal

• WWizards of the Coast2005-02-01
  Official Ruling
  Disrupting Shoal can target any spell, but does nothing unless that spell's mana value is X.
• WWizards of the Coast2018-12-07
  Official Ruling
  If a card in your hand has in its mana cost, X is considered to be 0.
• WWizards of the Coast2018-12-07
  Official Ruling
  If a spell has in its mana cost, include the value chosen for that X when determining the mana value of that spell, even if it was cast for an alternative cost and no mana was spent on X.

Rulings via Scryfall