Maui generally enjoys some of the best water quality on earth. Sometimes during winter storms on Maui, Molokai and Lanai water quality can be compromised temporarily by water runoff that carries excessive sediment, nutrients, and pollution called "Brown Water Events," characterized by brown-looking water. Plus, bacteria counts in the brown water can be elevated. Swimmers are advised to avoid areas with brown water runoff. Wash your body well after swimming in brown water.
In addition, Hui O Ka Wai Ola samples and publishes water quality data monitoring 24 locations in West Maui from the Pāpalaua Pali to Honolua and 24 locations in South Maui from Mā'alaea to 'Āhihi Kīna'u.
Hui O Ka Wai Ola (Association of the Living Waters)
Their website says they look for sediments, nutrients, and other pollutants from land-based sources that might be having a negative impact on water quality. They also explain that "Hui O Ka Wai Ola (Association of the Living Waters) is a group of water quality champions that is dedicated to measuring and sharing the status of Maui Nui's ocean water. Comprised of community member volunteers, scientists, supporters, and partner groups, Hui O Ka Wai Ola recognizes the value of a clean ocean for all of Hawai'i's residents, visitors, and natural resources."