Water Servant

Two activated abilities pulling against each other, both priced at the same single blue, turn a static 3/4 into a creature whose dimensions you set during combat. The body never grows: every point of power is borrowed from toughness and every point of toughness from power, so the elemental's total stays anchored at seven while you decide where that mass sits. The practical use is a blocking-math puzzle solved at instant speed. Facing one large attacker, run -1/+1 to pad toughness and survive a hit that would otherwise be lethal; needing to push damage through, pour blue into +1/-1 and swing for more than the printed three. The inverse coupling is what keeps the trick balanced: no configuration makes it both a wall and a threat at once, only a slider you push one way or the other, and stacking +1/-1 high enough drains toughness to zero and kills it outright. It is the kind of mana-sink creature that asks a blue deck what to do with its untapped lands once the early game has emptied out, converting a flooded hand into combat flexibility one mana at a time rather than offering a single big payoff. The flavor and the mechanic line up cleanly: a thing made of water has no fixed shape, only the volume you pour into one dimension or the other.