Being struck by boats is a leading killer of both animals, which are protected under state and federal laws.
The Georgia Department of Natural Resources says boaters should be ready to slow down or steer clear if they spot either animal. Manatees diving underwater often leave swirls on the surface.
Sea turtles as large as 300 pounds often only poke their heads out of the water.
Clay Georgia of the DNR's nongame conservation section says manatees have already been spotted this year from Savannah to St. Marys near the Florida state line.
Loggerhead sea turtles lay their eggs on Georgia beaches from May to September.