The park (in fact almost any park) is likely to be busy, manatee doesnt attract quite as many divers as some other state parks, but it does have a cap i believe (possibly even x per day rather than x in at the same time) - so it might be wise to get there early. Ginnie will most likely be busy all day as well, just have to muscle past the tubers and other non-divers to get into the water. From what i have heard from people i know enquiring about diving over that weekend, many places are already booked up for camping etc, so likely to be quite busy above and in the water.
As for nice to dive, apart from the people, its going to be high 80's and getting humid, so dont spend too much time out of the water, the water is going to be around 72 pretty much the whole year, not sure if some areas might warm up some more from the sun though, but not much i would guess. Have you dove either of these places before? Many here have and can give you a rundown on what to expect, what to do/not to do (particularly regarding lights for non-cavers).