Here's my opinion on the Flood card:
It should have a time counter on it, just like a Sundial, because a real flood doesn't last forever. I'd say it normally lasts 2 turns. However, for 3
, you can increase its counter by 1 turn, like a mage pumping water elemental energy to sustain the flood.
Both fields should be affected. This should be a global effect like Nightfall, and it balances the card a bit. Immortal creatures should NOT be affected; Flood is not a shield. Burrowed creatures should be affected. Water creatures are not affected because they can swim, and Air creatures are not affected because they can fly.
Lastly, Flood should not kill instantly. It could deal 3-5 damage per turn to all row one and row three creatures. That makes sense, because bigger creatures take longer to drown. Burrowed creatures should be killed instantly, because their holes are filled by water; Fire creatures should also die instantly, because they're vulnerable to water.
And creatures should definitely attack once before flood damage is dealt. Just like poison damage.