I'm not a local, but I can tell you that January weather is completely unpredictable. You oculd have warm afternoons with cool mornings, or you could have mornings near freezing with afternoons stuck in the 40s or 50s. And of course you could have anything in between. So you'll need to pack a little of everything.
Rain is not as frequent as it is during the warm months.
I'm sure you won't find many Floridians swimming in January, but you'll find lots of tourists swimming, especially those from the northern tier of states and Canada.