Your concept makes perfect sense however it would never fly because:
1. The ACLU will file a lawsuit because if you dig a moat and the Illegals cannot swim, this will interfere with their right to sneak in to our country. The access will be cut off preventing us from enjoying THEIR civil liberties.
2. If you use dirt to raise the levees, it will cut into funding to N.O. to improve the way of life there, and it makes too much sense, and if another calamity occurs the mayor down there will lose air time.
3. Putting gators in the Illegal moat as deemed by ACLU, they will piss and moan about it putting too much fear into the illegals that are trying to sneak in. This will cause traumatic stress on them, and prevent them from committing criminal acts, and jeopardizing law enforcement's right to incriminate themselves and not risk prosecution by sympathetic DA's. Not only that, if gators end up eating any illegals , their body oils may sicken gators and cause lawsuits by animal rights activists.
Maybe Jane Fonda, Tim Robbins, Susan Serandan, Sean Penn, Alec Baldwin, and other Far Left Hollywood entertainers should patrol the borders. They can give acting lessons to the illegals, that way when the illegals sue the U.S. Government, they can attain what many of us cannot afford.