God's divine capacities extend beyond omniscience. But in regards to His foreknowing of things, it is important to acknowledge, I think, that such foreknowledge doesn't necessarily equate to causation. It is this mistaken belief that it does, that foreknowledge must entail causation, that I most often run into when talking to unbelievers about the God revealed in Scripture.
God has made us in His image which means, among other things, that we have at various points genuine free agency. Of course, this freedom of choice is constrained by the way in which God has made us. We don't have wings sprouting from our backs that allow us to fly about like a bird, or the physical characteristics of a whale that would allow us to swim the ocean depths, we can't eat rocks for nourishment, or beat up a grizzly with our bare (pun intended) hands. About a number of things, then, we are constrained in our choices. But in the matter of the morality of our conduct, in regards to our response to the innate knowledge of God we all possess, we do have free agency and can choose to embrace God or deny Him.
No, I'm simply focusing on an important point concerning His omniscience that is commonly misunderstood. Such focus isn't an attempt to limit or ignore the other features of God's nature, however.
Well, I don't think "need" is the right word when talking of God's choice to flood the earth in judgment upon Man's wickedness. He could have used some other catastrophic means of punishing the evil of mankind, yes. But whatever He's chosen to do, as God, the Creator and Sustainer of everything, He has a unique divine prerogative to enact unilaterally. It's His universe; God is the Ultimate Sovereign within it; He can do whatever He wills in His Creation, which He sustains at every moment.