Almost all of the FP islands have decent places for snorkeling. That said the best will be on Rangiroa, then Tahaa, then Bora Bora ... then Moorea and finally Fakarava. Fakarava has a few places but the absolutely drop dead beauty is incredible.
Aitutaki has a few great placers for snorkeling but only accessible by lagoon excursion. Rarotonga only has one good place for snorkeling and again it is best by excursion.
Rangiroa and Fakarava are incredible for divers, world class.
As far as Resorts with best snorkeling it would be Le Tahaa Resort a Pearl property. Incredible snorkeling right next to it.
No major differences in FP between April and May. The Cooks can be a bit cooler in May than in April.
Tahaa drift area next to Le Tahaa Resort
Tahaa drift area next to Le Tahaa Resort - Bora Bora is in the background 14 miles away.
Tahaa drift area next to Le Tahaa Resort - drone shot, Bora Bora in the distance. Look closely after you enlarge and you will see the snorkelers
The Aquarium on Rangiroa
The Blue/Green lagoon on Fakarava - drone shot
The lagoon of Aitutaki