This one in particular deals damage equal to its power to a flyer that attacks you, but because it deals damage you can do all kinds of things with that, from lifelink and deathtouch to auras and combat tricks to force a bad attack
Maybe if Deep Water turned all your lands into Islands, it would matter for cards like Engulf the Shore, Scourge of Fleets, or Vedalken Shackles
If an effect says that an opponent may cast a spell during your turn, Tidal Barracuda's restriction overrules that permission
You have things like Quickling to protect your dorks while Cavern Harpy is on the stack and acts as important redundancy there
So Im a very enfranchised player, and if I hadnt looked up Commander ninjutsu ahead of time, I would have been uncertain about it when I first saw this card in a draft
This whole interaction of shuffling your library after you play Brainstorm is very common, and one of the main reasons to play this blue spell