On both the Dutch and French side of the island, there are some great snorkeling spots. The perfect snorkeling conditions are with no wind, calm waves, and bright sunlight. Preferably after some days of no rain or rough weather, those days the ocean is the brightest.
If you are planning to go snorkeling, please respect Marine Life and do NOT catch fish or touch the coral, one human touch can be enough for coral to die. Also, please buy reef-safe sunscreen and be cautious for boats around.
Coastguard does regular controls and there are big fines if they see you fish without a permit or damage an area.