The park has always opened in March. This is their Spring season, which is basically kind of a get employees ready for the peak season (Summer) and gives many locals a chance to visit weekends in Spring. Plus, they have to get those Spring Break crowds.
As far as full year goes, it has been rumored by many sources however they have yet to do anything to show us it may be possible. The only thing they have done to make this possible is adding Christmas Town to extend their season, which could lead to an extension into other months. The problems they face with full year operation would be maintenance work, seasonal employee contracts, and actually getting people to visit in the Winter.
I am sure the first two are easier to deal with because Tampa handles those just fine, but that last one is very difficult. I am sure they could manage up a few crowds on certain days mostly composed of locals, but I'm not sure things would work out too well.
I do think they could get away with weekends for locals and have a smaller staff, and they just go for the locals that time of year. I mean yeah it could be cold but you will always have those crazy people who want to visit, or those troublesome teens who want a fun place to go, or something of the sort. These mainly consist of locals who purchase season passes, etc.
Still, weekends in January and February could work out, if they can get enough locals to visit and they use a smaller staff.
So in conclusion, no the park has not made any notion of going full year as of now, however they always open mid-March for their Spring season.