Sounds like the manatees gather in the warmer discharge water from the power plant near VB.
Homossasa is about an hour or so north of Tampa.
A great location to view manatee in the winter months is at Blue Springs State Park near Orange City, just off I-4, north of Orlando, on the way to Daytona. Many manatees come into the spring area because the St John's River gets colder than the spring (where the temp is about 72 year-round) - the water is clear and they have walkways above the spring and the spring run to view them.