To catch a great White Shark in Animal Crossing: New Horizons, you will need to look for a fish shadow in sea and with visible fin.
You can find the white shark in Animal Crossing: New Horizons as of June 1 until September 30 in the Northern Hemisphere from 4 p.m until 9 a.m in the sea.
The Great White Shark is only active from 4 p.m. until 9 a.m., while fish like the whale shark are active all day. Try using many fish baits for more opportunities.
As with other sharks like the whale shark, you can catch a great white shark by fishing in the sea and looking for a large fish shadow with a fin. The Great White Shark is the rarest to locate in the sea, so you may end up catching a Suckerfish, Hammerhead Shark, or Whale Shark before encountering the Great White Shark.