You don't need to rent equipment or spend on a special tour to see some of the best reef life in Hawaii. Much of the colorful coral is accessible from shore, including the majority of locales that are also reached via boats with paying customers. Hundreds of coastal reefs and beaches offer decent to excellent fish bowls—provided, of course, that the ocean cooperates with low surf. Exceptions: Molokini Island off Maui and Lehua offshore Kauai are two memorable tourist snorkeling experiences. And, scuba diving companies take people to numerous spots that are too far offshore. Trailblazer guides have dozens of snorkeling beaches on all the islands for the adventure traveler; safety tips are included in the text, as are spots fequented by tour companies.
If you don't pay to snorkel, then you are going to need fins, mask and a snorkel. If you don't already own a set, then for sure wait until getting to the islands until finding one. It makes little sense to rent equipment, since it's cheaper to buy if you are here for a week. All sorts of cheaper-than-renting equipment is available at ABC Stores, Whalers', Long's Drugs, and Wal Mart—places you will probably frequent to stock up anyway, and then won't have to worry about returning stuff when leaving. You can spend around $25 to $30 and be good to go.
Also, many vacation rentals—from individual condos to big resorts—have gear on-site for free. Ask them before signing on the dotted line. Check out Hawaii's best places to snorkel on Maui, Kauai, Oahu, and the Big Island. If you want them categorized and are still trying to decide which island to visit, buy a copy of the No Worries Hawaii guidebook.