Summer, definitely. Also winter whale season, but summer is highest. Christmas and New Year's weeks are also very high season, with kids out of school.
...If the snorkling is poor, we will try early May to mid June for a change. Prior to my retirement, we vacationed in Hawaii for several weeks during prime time, including the July 4th holiday. :whoopie:
Assuming snorkel conditions mimic diving conditions, my experience has been location is more a factor than time of year. Maui and the Big Island are the best locations as there is less sand and more volcanic rock allowing for greater visibility. Kauai, in my experience has been the worst as it is the oldest island and has more sandy bottom conditions which cuts the visibility(20-30 feet vis is not unlikely whereas a bad day on the Big Island is 50'.)