Hey aussiebossyboots, I haven't snorkelled from Noumea, but have enjoyed the aquarium, the tchou tchou train and the hop on - hop off bus at various times. There is also a Patisserie one block to the right of the cruise terminal that will spoil you for ham and cheese croissants for life!
Lifou: OMG don't miss the Jinek Bay pass! We spent about 2.5 hours in the water there, we ended up right out at the drop off, which we were told afterwards usually results in a whistle and call back but didn't for us. So many fish and critters among the coral and outcrops, it was fabulous!
Mare: head for the beach and take your snorkelling gear as there are some good areas, depending on the tide you may have to head out a fair way but it's all protected inside a reef system.
Isle of Pines: We were there at the beginning of May and did do the Turtle Bay and Brush Island excursion. We only saw 1 turtle in Turtle Bay but saw others along with rays and fish at Brush Island. To be honest, I would be more inclined to snorkel around the sacred rock, as long as the weather is kind, it is a nice swim to go right around it and there is plenty of wildlife to be seen. There are also local tours available for about $30 per head which give you a good tour of the island for a couple of hours. I found the old prison fascinating and could have spent lots longer exploring!