Miami-Dade has issued a No Contact with Water Advisory for marinas in Virginia Key, Key Biscayne and Fisher Island.
In a press release, Miami-Dade's Water and Sewer Department says a temporary electrical power outage at a treatment plant resulted in an overflow of chlorinated sewage that went into the waterway.
Power was restored within an hour, but 100,000 gallons of the treated, chlorinated sewage reached surface water, the department says.
Miami-Dade asks residents to avoid swimming, fishing and boating for all beaches within Virginia Key, Key Biscayne, and Fisher Island.
The advisory will stay in effect until there is two consecutive days of clear testing, the department says.