They say no permits have been issued for the two-day event, but a Facebook page promoting it has received more than 14,000 "will attend" responses.
Floatopia has exploded since it began six years ago with several hundred people drinking offshore on rubber rafts and inner tubes. In 2009 the event attracted as many as 10,000 revelers in what the county parks director described as "unmitigated chaos."
Last year there were no major incidents after authorities closed some beaches and increased police presence.
The Associated Press contributed to this report.