Just thought I would tease you with a picture of Malamala Island. This is an excellent day trip if you are on the Nadi side of Fiji. It's not really expensive, it's a pleasant boat ride from Port Denarau, and the food is fantastic.
Might be a bit scary if you are a first-time snorkeller and aren't too comfortable in deep water. You get taken by their little boat out into the deeper water and are told to jump out. But if you are happy to float along it will be a fun trip.
If you do decide to go on this trip, take the smart option – go for the Earlybird trip. It is a slightly cooler part of the day to travel and normally there are fewer people on the boat. It also means that you will probably get more choice in equipment if you aren't bringing your own. If you are a diver/snorkeller, try to bring your own mask & snorkel as it will save a lot of hassle dealing with the older equipment available. The fins they have are just fine.