This trip was a real highlight of our holiday to Australia - we did it on Christmas eve. Half day was plenty of time snorkelling, we had a guided snorkel and then were able to go out on our owns. We saw 2 turtles swimming very close by. The tide was high when we went, so we were quite high above the coral which meant the colours weren't as bright as you would expect, but still a great experience.
We chose half day and I'm glad we did, I suspect we didn't miss much as there was plenty of time and fish to see. The staff were good, after snorkelling we had tea/coffee and snacks.
There was a glass-bottomed boat available if people weren't confident to snorkel but I recommend the snorkelling where possible, as you really experience everything and are close to the fish.
If you do a full day trip you have a longer journey to the further reef. This was just a half hour out, so if you get seasick or not keen on boats pick this half day one!