At low tide you can walk and wade along a sandbar from the Pacific Resort Aitutaki right out to the reef, but wear something on your feet to protect yourself from the coral, sea urchins, eels, stonefish, algae, etc. Stonefish are not common, but they're almost impossible to spot until it's too late.
At high tide snorkel out from the black rocks on the beach just north of the resort. Snorkelers and paddlers must keep at least 200 meters inside the main reef entrance at Arutanga because of the strong outgoing current.
The finest snorkeling off the main island is at the far west end of the airstrip and to the south. Beware of dangerous currents in the passes near the edge of the reef here. Elsewhere on the main island the snorkeling is poor.
The top beach on the main island is at the southeast end of the airstrip near the bridge to the Aitutaki Lagoon Resort. It's fine to swim here, but snorkelers won't see many fish or corals.