Talk to your realtor. They're likely being sued as well, they've got E&O (Errors and Omissions) insurance for this very case.

I talked to my realtor, not that she's involved in this or anything. She said:

The buyer needs to prove (as was mentioned) that it's happened before and that you knew about it (withheld the information). If the house never flooded, they'd be hard-pressed to prove it happened and you won't need a lawyer.

I've actually known a couple whose house flooded every time it rained in multiple rooms. They did indeed withhold the information, and were indeed sued.. unsuccessfully. It's really hard to prove that you knew.