If you have found yourself in Gansbaai then odds are you are here for one thing - Great White Sharks! Gansbaai is one of the main places in South Africa where you can go cage diving with the world's most feared creature.
There are many companies offering Great White Shark cage diving services in Gansbaai, and most will also include breakfast and lunch (you will want lunch especially after a few hours in the ocean), and all at very inexpensive prices.
You'll be out on the boat for a couple of hours, and the water can get quite rough so make sure you have sea sickness tablets to hand if you suffer in any way from motion sickness.
From what I understand almost on every trip out there you will see Great Whites. On my trip I saw 6 or 7 different Great White Sharks, and there was never a long period without seeing them.