I like the concept. Flood is a card I'd almost never use as it stands. Giving water more reason to actually play it would be cool.
As for how powerful the creature is; you've got a maintenance cost of 3 water per turn, and I'd assume by its description that if the flood gets destroyed, the fish would die so you'd practically have to base an entire build around aether/water, and a creature that without buffs dies to a lightning storm. Outside of that, as it was already pointed out, you need 7 creatures on the field to force the thing into a flooded row (unless the card survives regardless of placement).
Either way, it would still need work and some serious testing, but not a bad idea giving flood more use.
