When buying rental car insurance:
Don't bother checking the "yes" box and paying for Collision Damage Waiver (CDW) coverage if you've got a credit card that offers that same coverage. Most credit cards provide that insurance to their card holders free of charge as long as you use their card to pay for the rental. A quick call to the customer service number on your credit card(s) will fill you in on exclusions and protection.
Do bother checking the "yes" box for liability insurance protection if you don't have your own auto insurance, or if your own policy has only minimum coverage for injuries and medical bills. If you don't have comprehensive or collision in your own auto policy, paying for CDW does make sense. But if you're renting for more than a few days it may be cheaper to add those back into your auto policy. CDW charges are no bargain.
When buying flood insurance:
Don't bother trying to buy it in the middle of flood – there's a 30 day waiting period before it kicks in.
Do bother continuing to pay your NFIP premiums if you have flood insurance. If you let it lapse, you'll lose "grandfathering" protection and may get hit hard this October when rate increases passed by Congress take effect. If you've been thinking about buying flood insurance, do it now!