Access just past the Renaissance hotel in Wailea, This beach is, for most, the best snorkel beach on south Maui. Stay to the right an follow the reef out. Don't stray over the shallow reef to far the surf can push you into the rocks and the sea urchins are a bit sharp. The turtles are endangered and protected by federal law. Ridding a turtle can get you a free place to stay on Maui.
Ahihi Cove is a little harder to find. Drive South pass Makena State Park, the road narrows to nearly a single lane so do yourself and every one else a favor and don't take a wide vehicle. The road opens up to a small cove just before the Lava fields, That is the spot. There is the remnants of an old launch ramp on the right side of the cove that makes for a good entry except at low tides.
Old Airport or Kahekili Beach in Kaanapali is your best bet for snorkeling on the upper west side. It's north on highway 31 and left onto Kai Ala drive opposite the Sugar Cane Train. The reef starts as soon as you set foot off the beach. Stay deep enough that the surf doesn't grate you on the coral. It will damage the coral and it doesn't do you any good either.