After snorkeling, it's great to have some snorkeling fish guides, to figure out what you saw! We own a variety of snorkel fish guides, for Hawaii and the Caribbean, that help us identify the wonderful marine life we find when on our snorkeling trips.
Probably the best guides available are by Paul Humann and Ned DeLoach. Their Reef Set series has been the standard for fish identification for many years. If you are avid about the underwater world like us, do yourself a favor and buy their full Reef Set. This boxed set includes, Reef Fish, Reef Creatures & Reef Corals. This set is focused on Florida, Caribbean & Bahamas.
The same authors also put together a nice condensed book specifically for the marine life snorkelers will most likely encounter. We have used ours so much it is falling to pieces, and it is generally excellent.
At the same time we bought the Reef Set above we also got their book Reef Fish Identification - Tropical Pacific - for help with Hawaii fishes and for when we explore more of the Pacific Islands.
For Hawaii specifically, you can't go wrong with John P. Hoover's Hawaii's Fishes: A Guide for Snorkelers and Divers.
Another good option for Hawaii is John E. Randall's Shore Fishes of Hawaii.
And if you want to complete your guides to Hawaii, consider also getting Corals of Hawaii, by Douglas Fenner.
Have fun learning about those beautiful fish and corals!