Cindy, I just realized you said you're going to the Great Barrier Reef in January. A wonderful place! I do want to caution you about the stingers that are active in the waters that time of year, in case you haven't already made the arrangements to deal with them.

A lot of the snorkel trips don't run during that time of year because of the stingers. These aren't your garden variety jellyfish--their bite is supposed to be extremely painful, and can lead to death.

Precautions can be taken. We went in October, which was the start of the stinger season, and all snorkelers were required to wear stinger suits. Think head to toe spandex.
The week we were there was the last week that our snorkel/dive boat would be going out until after stinger season.

I understand that there is a pontoon with a snorkel area enclosed in nets to keep out the stingers, but I've not heard good things about the experience there.

I expect there are other options for a good experience that time of year, but I do want to caution you to do your homework ahead of time to make sure you have a good experience, if you haven't already done so.