Might need to change the wording a little.
It says "and cannot be killed" ... which many might think means they become immortal.
Probably better to use "and will not be killed by flooding"
Also, it might be good to put "ally creatures" in the first line instead of just "creatures" as it is now, unless you mean for the opponent's creatures to be affected as well.
Other than that, I think this card should be good to go.
If it makes it in, flood decks will become much more viable. I think it would definitely bring something new to the game since flood is currently very underused as far as I can tell.
Gives me lots of deck ideas... could be very fun! I really like it.