Concierge.com's insider take:

There are many, many spots with incredible visibility—up to 100 feet—along the west side of the island (even right off the Mauna Lani and Mauna Kea resorts). There are also two incredible swimming and snorkeling spots on the southeast side. The first, Champagne Pond, is a calm ocean cove heated by the volcano. The water is also slightly effervescent, but that doesn't seem to bother the abundant fish and turtles. It's accessed from a dirt road off Highway 132 (heading toward Kapoho from Hilo); a four-wheel-drive vehicle is a must. At Kapoho "Wai Opae" Tide Pools, a collection of spring-fed brackish pools, beginners will feel safe and even the most jaded snorkelers will be impressed by the coral and fish. To reach it, drive to Kapoho at the end of Highway 132, then south on Highway 137 to Kapoho Kai Road.