We have always enjoyed snorkeling and when the water's calm, it is nice to snorkel right off the shoreline . . . assuming you can get to the water. The Big Island has a lot of coastal area with very rough entry due to the relatively recent lava flow activity there.
If you want to snorkel on your own time and schedule, we would suggest buying the books "Snorkel Hawaii" and "Hawaii The Big Island Revealed'. Both are outstanding resources and can probably be picked up at your local Half Priced book store. If you really want to only deal with one book while on vacation, the Revealed book has more general information and greater utility.
When had always heard about the Puna tidal pools but didn't really venture too far off the beaten path (i.e., into the neighborhoods) to find a nice swimming area. Some of the tidal pool areas are very shallow, too shallow for even the smallest adult. Since David and I displace quite a bit of water, we need to be sure to have some depth or else we will scratch ourselves up on lava rocks and coral.
At our B&B we asked Petra for directions to the Kapoho "beach". I use the quotes because there really is no beach in this area. It is a row of waterfront houses on a street that dead ends into a rather new parking lot, that immediately places you on the lava rocks. As you can see from the photos attached to this review, they are very jagged and unless you fall on them, you have no idea just how sharp they are too! I tore up my left hand pretty good just getting out of the water after our snorkel swim.
Here at the Kapoho pools, you will need to walk out about 10 or 15 yards out from the shore to get to a place to launch into the water. Prior planning will be your friend here, as you will want to walk out on the big lava rocks in protective reef shoes. If you snorkel like we do with the less expensive fins, you will need to have a place to take off and leave your reef shoes when you put on your flippers. Many people don't care much for that hassle so they have the fins that go over the reef shoes and strap around your ankle. In either event, do not attempt to walk on these rocks barefoot or in flimsy flip-flops.
Once you are geared up and ready to go, you will be pleased with the wonderful marine life here. With the area a designated conservation area, there is no fishing which allows for added development of the underwater world beneath you.
A young reef, there are some very interesting coral here including large scallop coral which I had only seen previously in photos. The fish were also very unique in this area - fish we'd never seen before in Hawaii. I especially like this rather large porpoise faced blue fish (photo attached). We also saw a couple of unicorn looking fish, with what appeared to be a horn type protrusion right out of their forehead.
With the shallow areas throughout, there were also some pockets of schools of small nursery fish. It was cool to see tangs and some of the other Hawaii favorites that were no larger than a dime or nickel.
If you do not own your own gear, you can rent the full set up (mark, snorkel and fins) for around $10/week from either Snorkel Bob's or Boss Frog. During our time in Hawaii, they both were offering rent one set; get a second for the same rental period for free. You can't beat that, although we prefer to own our own since sucking on plastic that was in someone else's mouth yesterday is squicky to me. I'm sure they use industrial strength disinfectant; I just don't want to provide an opportunity to suck the germs from a perfect (or not so perfect) stranger.
The local snorkel shops in Kauilu-Kona will also give you good tips and advice on places to go if you want to do a do-it-yourself type snorkel day. They are also more than happy to book you on a tour which will set you back around $100 for the morning and $75 for the afternoon. NOTE: It is more in the morning because the waters are typically calmer and clearer making them the more desirable trips out. Afternoons can have windy, choppy conditions that churn so much junk up your visibility will be hampered and your photos cloudy.