Most of the beaches are separated by rocky outcrops that jut into the water and when they submerge turn into a reef. These outcrops shelter the beach, making it good for swimming, and make the edges more interesting, making it good for snorkeling and fish.
The rockier beaches are farther south, near Makena and particularly La Perouse Bay. Most beaches have some sort of a lawn behind the sand, particularly at Kamaole 3 (South Kihei) and Mai Poina Oe Lau Park, north of Kihei. Some of the beaches, mainly those listed on the map as a park, also have a restroom and showers. The best snorkeling is in sheltered bays, with lots of rocks. This would be at La Perouse Bay, but that end of the road is pretty rocky.
Maui Hawaii Travels 2002      
The southwest coast of Maui is where most of the good swimming beaches are. The roads parallel the shore and for ten miles is a drive past one great beach after another.