Cairns is located in Tropical North Queensland so the water temperature is very pleasant all year round. Approximate average water temperatures throughout the year are:

• December-February: 29C or 83F
• March-May: 25C or 76F
• June-August: 22C or 72F
• September-November: 25C or 76F

There are two snorkelling areas around Fitzroy Island. The Nudey Beach area can be explored at high tide as well as at low tide. The water entrance is from the beach. 

However, snorkelling at the dock area is tide-dependant. In fact, at low tide, it is often impossible to get into the water without stepping on the coral (which is forbidden and dangerous). Make sure to check the tide times.