South of Kihei and Makena in the Ahihi-Kinau Natural Area Reserve, there is good snorkeling in La Perouse Bay. The entire region is within lava flows from the 1700's. The lava fields flow right down into the bay.
When snorkeling at Le Perouse, you may encounter a pod of spinner dolphins that frequent the waters here. The bay is a resting place for the dolphins, so make sure not to disturb them.
This a challenging place to snorkel, even when the usually high winds are calm. The best snorkeling is to the north towards the waters off the Ahihi Reserve. Keep in mind that much of the land within the Ahihi Reserve to the north of La Perouse Bay is closed.
To reach La Perouse Bay snorkeling from Wailea-Makena, head south from Wailea along the coast on Makena Road to the very end. There are two dirt parking areas. To reach the snorkeling on Maui by air, you will need to fly into Kahului Airport (OGG) which is located in the middle of the north coast of Maui. It is served by direct flights from the mainland US and also by inter-island flights from several other Hawaiian Islands by regional carriers.