Pay attention to official warnings about beach conditions. A green warning flag means it's safe to swim. "No swimming" (red) means just that. "Caution" (yellow) means hazardous conditions such as riptides—ask the lifeguard what dangers lurk. A purple flag means "dangerous marine life," usually referring to Portuguese men-of-war—jellyfish that look like blue bubbles floating in the water, and whose tentacles pack a mean sting (if you get stung, head for the lifeguard, who'll treat you).
Sea lice are more annoying than dangerous. They latch on to your swimsuit and cause itching. To relieve itching, shower with soap and water. For information on conditions, call Miami Beach Patrol (305-673-7714).