Apart from monsoon that happens during the months of June and July, any other time is best to visit places in Andaman and do snorkeling or diving.
January to May is the period when the sea appears calm and the surface of the sea would be relaxing. During this time, diving or snorkeling would be done smoothly. The crystal clear water will make your way easier.
From the month of September to November, the natives call it new season, the conditions would be highly favourable to do diving and snorkeling. The winds are unpredictable this time and also regular showers will happen which lead you to limit the activity to certain limits but still most of the day in these months, the climatic conditions are preferably good to do snorkeling.
December also is considered to be a favourable time to do the water based activities though wind and us ual showers will be there. This period is called the transition time but still helps tourists to have a great time on the water.
Anytime you visit, you will be able to take part in snorkeling in Andaman unless the climatic conditions are so rough. If you are visiting the island during on-season, the days would be sunny and the beaches will be filled with tourists. But during the off season, the shores are quiet but the climate would be unpredictable.