The whale watching season runs from May to November, when southern right whales arrive to calve in the waters off Imbituba. The peak is July and August. You can often see them from the shore — particularly from the headlands around Praia do Rosa — without needing a boat, though boat trips are available.
Summer (December to March) is beach season. The town fills with visitors from across Santa Catarina and further afield, the water is warm, and the surf is generally smaller. If you want the beaches without the crowds, come in April or early May — the water is still swimmable and the whales are just beginning to arrive.
The Festa do Camarão (Shrimp Festival) takes place in January and centers on the local fishing tradition. Not a spectacular event, but a good snapshot of what the town actually is when tourists aren't the main event.