I live on Oahu and have snorkeled quite a bit. The West side of the island can be a little more "tricky" to safely leave your belongings on the beach, but the snorkeling is incredible. We wanted to snorkel spots we had not been and be almost guaranteed (knowing that with wild animals, there is no such thing as a guarantee) that we'd see and hopefully swim with dolphins. We ended up that day snorkeling a spot we had snorkeled before because that's also the area where the dolphins were. We knew there was a chance we wouldn't snorkel somewhere "new", so that was no problem. We swam with dolphins all around us. It was AMAZING. The crew was awesome. (They won't say anything about this, but a tip at the end of the tour is a nice gesture given how great they will treat you.) The photographer is AWESOME and you'll have great pics to show your friends and family. She will show you a small album of her work, so you have an idea of what you'll be getting. The boat is not huge, so you won't be on there with tons of people. To me that's important. We were comfortable, it's very safe (although, you're talking about open ocean and swimming with larger sea creatures, so I wouldn't bring a small child), the staff/crew are so friendly, and we had so much fun. Give them a try. I don't think you'll be disappointed!
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.