Water Purifier: Since all your neighbors' stank will be floating in the water, you'll want to use a purification system before you take a swig. Get a small and compact one, as well as some empty jugs to put the water in.
Waders & Boots: You'll be trekking through water of varying levels, so have some waders and waterproof boots on hand. Waterproof socks are also great items.
Radio: More storms could be coming and more dams could be breaking. You'll only know this if you have word from the outside world. Make sure your radio is waterproof, like this submarine radio, and pack extra batteries.
Matches: Obviously get some in a water-resistant box. You never know when you might be able to strike up a fire for warmth, cooking or signaling rescuers.