As a S. Florida restaurant, and previous Miami Beach resident, I can assure you that most restaurants there do automatically include the 18% tip and most give minimal notice to the customer that this is being done. Every bill I was ever presented with still has the area for "gratuity" and if you're weren't paying attention, you would probably pay an additional tip.

As an aside, be VERY careful with credit cards there. I generally try and pay cash for everything on the beach as I have had my credit card information swiped 3 times now. (Fortunately, because I am so paranoid, it was my corporate card - I was on business). I did however find a creative way to dispute the charges on one... I arrived home one trip to see that Victoria's Secret had declined my order and needed to speak to me immediately... I sent them a picture of my shorts clad behind and assured them that I wouldn't fit into a size 0... I never heard another word.