whoever said you had to scuba to see fish and coral in hawaii was wrong...i learned to snorkel at hanauma bay and saw tons of fish and coral in 3' of water....all the colorful fish and coral are usually in the top 50' of water and with great visability you can snorkel and see it all.
We havent cruised Hawaii but have stayed many weeks on different islands and have snorkeled most of the prime spots...for a cruiser the main problem is lack of transportation, only ohau has busses and taxis..there are tons of places to snorkel right off the beach or the side of the road...the snorkel rental places give out maps that show tons of spots for whichever island they are on...hanauma bay is the best spot for beginners on ohau (very crowded with tourists, but worth it) multiple tour companys will take you there...for maui the best spot is molokini which is one of 3 spots of its kind in the world, its a volcano top barely out of the water with the leeward side open to the ocean...most of the year it provides a sheltered area to snorkel and most every boat has a tour there...there is a steep drop wall dive for scuba people on the windy side, awesome place, just make sure the wind isnt blowing into the crater like it does sometimes in the winter months when alternate sites are better, maui gets whales in winter months. search for hawaii activities and you will find a few of the big companies that run all kinds of excursions, most offer snorkel boats on most all of the islands. I think they would cater to the cruisers and provide transportation to/from pier, but you have to check and make sure.
kauai's poipu is a nice sheltered beach, a good place if you have small kids, but snorkeling was sort of limited there...not bad, but not "the best"