Florianópolis is quite spread out so plan your stay carefully especially during high season when traffic can be very lintense especially in Lagoa, the eastern beaches and on the roads to the north of the island.
After peak season driving a car gives you freedom and flexibility to explore the island.
The public bus system has nine terminals. The terminals are like hubs for bus services connecting different regions. So let’s say, if you are travelling from city centre to Mole beach, you get a bus to Lagoa Terminal and then get another one to the beach. It isn’t the best public system but you can get nearly everywhere. If you do not speak Portuguese find a friendly local check your route. For timetables please check out timetables (only in Portuguese).