This is mainly for the Brits and any others coming from outside the United States, like California or New York:
Mobile is NOT Hawaii. If you choose to wear a Hawaiian shirt, just be aware that only some of the locals will be wearing them as well.
Americans no longer wear coon-skin caps. If you wear one, make sure it is CLEARLY fake-fur to avoid entanglements with animal-lovers.
The official langauge of Alabama is kinda like English--at least somewhat related to American English.
If you choose to speak in your native accent, don't be alarmed if the locals think you are French--unless they are still angry at the French. Then remember that they are Americans, and be alarmed...be very alarmed.
Passports are not required for travel between states. If anybody asks you for one, he's not very well versed in the law and can be safely ignored.
Mobile is probably not a 'dry' town, so there will not be any need to cross county-lines on a beer-run.
I hope these tips are of some help to you.