A very nifty card indeed. (How is it fair that I'm seeing all these cool environmental ideas for
all the time?)
Combined with flooding, and the opponent is nearly screwed when it comes down to playing a mono/duo deck filled primarily with water cards in it. It honestly shouldn't kill targets, as some elements (see Fire, Time, Darkness, and Entropy) would stand no chance to play cards on the field with that effect out - although I realize that Fire can destroy permanents. The Maelstrom's +1 AP to the water creatures is a nice touch, and it's good that you put an
increased cost for that buff.
This effect is similar to Nightfall/Eclipse, in that it cannot stack if multiple ones are played onto the field, correct?