Manatee, like most (all?) Florida springs, is a nearly constant temperature all year long. Most in th e"cave country" have temps of about 70F, definite wetsuit conditions, but I am quite comfy in a 3mm suit with 5mm hooded vest for cavern/cave, and basin diving is usually OK with just a 3mm wetsuit(maybe a 3mm hood). Be warned that travelling beyond the basin could expose you to river water, which is much colder in the winter (well, probably only down to about 60F, but that feels really cold when you've gotten used to 70!). I remember being over Devil's Ear in January a few years ago and gasping hugely when I drifted into the browner and much colder waters of the Santa Fe river.
I haven't tried very hard to see Manatees in the water, but I don't believe Manatee spings actually has that many manatees anymore, if it ever did. That would be for someone else to answer. I think Crystal River is the best venue for manatees.
Good luck and hey, if you're there near Christmas, maybe we can dive together! PM me! I am itching to get wet again!
As for the general weather, we central Florida folks say if you don't like the weather, wait 5 minutes! It could be 30F or 80F, and you probably won't get a reliable prediction until a week or two before the date, and then only "It'll be cold" (30-60) or "It'll be warm" (55 and up) until you get to the 5 day forecast that is always SO accurate!
