According to Shark Diving Unlimited, based in Gansbaai:
Low season
January to March, however shark activity still continues and you can
come and see Great White Sharks during this time.
High season
May to October is the best time to see the sharks.
Winter is better than summer
Our water is warmer in winter and the visibility better. Winter is
also the time when the sharks hunt around the island for seals.
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