Based on the fact that the water seemed to be pretty cloudy there, I would say that the snorkeling at the Aitutaki Lagoon Resort is not that good. Also many of the units there are long overdue for an upgrade (some of them needed it 3 years ago when I was there). General agreement that the place is overpriced and in poor condition (at least for some of the units).
West side of Aitutaki from around the Pacific Resort north to the airport has generally good snorkeling. Know we liked it very much around the Are Tamanu.
On Rarotonga, best snorkeling area is usually considered to be from the extreme southern end of Muri Beach, down around the south-east corner of the island. We stayed on the south/central shore near the Palm Grove and enjoyed the snorkeling very much - so much so that we are returning again to this very are next April
North shore has really no snorkeling as the reef is up against the shore. This stays true down the east side until you come to Muri Lagoon.
West side snorkeling is OK but the reef there is much closer to shore and the water a bit shallow.