Brine Hag

Let's dispense with the explanation and jump straight to the card this time.

So, let's review: It's essentially the same card, but it's in white. Why the color change? Because, of all the colors, blue has the most control over how its answers affect the board. It's a subtle difference, but note that the abilities on most modern blue creatures either target or trigger on combat damage to a player. (That's intentional -- blue has more ways than any other color of sneaking attackers through.)

Brine Hag, however, requires quite a few things to go right for the blue player. First, the opponent's duders can't fly, can't have fear, can't have islandwalk -- in short, they can't have any evasion. Blue's not the color of robbing opponents of evasion, Tolaria notwithstanding (edit: I misremembered Tolaria as Urborg and/or Hammerheim), so that's an unusual requirement.

Second, blue and white both have strong defensive themes. However, look at how white defines defense -- it runs out high-toughness creatures to block up the field, robs opponents of their options via effects such as Humility, and blows up whatever's left via Disenchant, Swords to Plowshares and the like. Blue, however, stalls opponents via bounce or power-reducing effects, counters "can't-deal-with-it" targets and uses generous tutor effects to grab the just-right artifact or spell for the job. Brine Hag stalls and reduces power well enough, but its ability looks a lot more like Crib Swap than Boomerang. Moreover, a staple of many white defenders is the archer card -- cards like Ballista Squad and Crossbow Infantry that tap to deal some amount of damage to attacking or blocking creatures. Those cards are almost entirely absent from blue's portfolio. Wall of Brine is in the same mold as one of those archer cards -- it has a known, limited effect that has a quantifiable effect on the field.

Of course, there's the blue version of this card.

It's far more versatile, but it has less powerful control element -- unlike white, blue has a harder time permanently impacting the board. Blue's also been given more and more power-reducing effects in recent sets, including ones like Turn to Frog that set power to a specific point.