The Grotto is indeed the jewel of Ontario. This scenic cave which contains a pool of blue water is located on the Georgian Bay shoreline in Bruce Peninsula National Park, not far from the town of Tobermory. The cave is hidden in the cliff face and its secluded beauty has been attracting visitors from all over the world. The Grotto Parking is available from mid-May to late October, the summer months of July and August can be very crowded. So your best option is spring or fall.
The Grotto was carved from the natural stone by wind and water over thousands of years. It's part of the scenic Bruce Trail at Bruce Peninsula National Park. The Grotto is located at the Cyprus Lake Road entrance to the Park. It can be reached from the cave mouth at Lake level or after you walk a short 45-minute rocky trail. The water of the grotto is so blue thanks to the light that emanates from an underwater tunnel. The Grotto is not a good place for swimming because water is always cold, although some swimmers try it out anyway. Scuba diving is also possible.
The Bruce Peninsula park is famous not only for The Grotto but also for bears. So watch out for wild animals during your hike.