"What I don't understand is I have a pump that works under water."
Your pump probably runs on electricity. If the electricity is off, your pump won't work.
The large pumps on the levees, however, run on Diesel, for the most part. Diesel engines don't run underwater, either.
It's a real problem.
According to Fox, some of the pumps do work on electricity, some on diesel. The diesel ones will never work while under water. And with no electricity, well the electric ones won't work either.
But with advances in technology they need to re-think that whole set up.