I stay in Cairns in Caravella Backpackers – they have easy booking via internet and they provide free transport from the airport.
Many competing companies offer day excursions for the snorkeling on the Great Barrier Reef. I took a catamaran departing at 8 am and retutning at 16.30.
They took me to two places on the reef… I did snorkel here years ago, but these were new sites and what you can see under the water is just wonderful.
They provide snorkeling gear, breakfast and lunch are also included. Snorkeling you have to protect your back from the strong sun… Great day.