Big Beach is one of Maui's largest beaches (roughly 1100 yards long and 65 yards wide). Part of the allure of Big Beach is that it's located in the undeveloped 160 acre Makena State Park, so it feels remote and wild compared to most Maui beaches. Although a popular boogie and body boarding site, serious injuries are a weekly occurrence thanks to a deceptively forceful shore break. Inexperienced swimmers and boogie boarders should stay out of the water (there are much safer swimming beaches nearby). Strong currents are also common - fortunately the county now has a life guard tower at the beach. The southern section sometimes offers calmer conditions and is also less crowded - this section can be accessed from the second parking lot.
Over a small hill just north of Big Beach is Little Beach, Hawaii's most famous nude beach (and one of only a handful in the state). Although technically illegal, authorities don't bother nudists when they stick to the known nude beaches. Little beach has better swimming conditions than Big Beach, but it has no life guard coverage. Also, most people will feel uncomfortable swimming with the nudists, most of whom are local residents.
Note that Makena State Park has specific hours and charges an entrance and parking fee. You can find current information about rates and hours from the Division of State Parks site.