Hilo Ocean Adventures is the main tour operator on the Hilo side of the Island. they offer snorkel, scuba, whale watching, fishing, and waterfall tours. To to https://www.hilooceanadventures.com for more information.

there are many more tour operators on the Kona side that do all of the above, plus manta ray excursions and sunset dinner cruises. A quick search will find you many choices.