Being divers, the GBR was a must-see during our trip to Australia. We did a lot of research online for different operators that did diving tours and decided on Poseidon Cruises.
I can't even remember what the decided factor was for this. I know we were contemplating Silversonic as well. We really wanted to go for Silversonic's dive-only vessel, Pure Dive, but the price was just a bit too high.
Our experience with Poseidon was fine. They picked us up on time, got us settled in, had anti-nausea meds on board, fed us snacks (coffee, tea, cookies, muffins), and fed us lunch (bread, cold cuts, prawns, veggies and fruit). If you want to save $3, bring your own meds, very absorbant towels as the indoor area gets very wet, and anything else that might make you feel less nauseous on the boat trip out. It's a long boat ride and can get VERY bumpy. I wish I had brought my Tiger Balm because the strong smell of mint makes me feel a little better. It can also get very cold and windy (esp while the boat is cruising) so a fleece would be a good idea.
Oh, if the sigh of other people getting sick makes you sick, um, you might want to stock up on their biodegradable barf bags.
We had 13 divers on board, and maybe 30 snorkellers. I think the numbers can quadruple in high season. They divided the divers up in to 2 groups so I was happy about that. Our dive master was a nice guy and was quite thorough in his explanations. He kept an eye on all of us divers and went at what I felt to be a pretty good pace, although maybe a bit too fast at times? The husband likes to really take his time to look at everything around him (and he almost always spots something neat!) so I felt like I was trying to keep up with the DM but had to keep looking back over my shoulder for the husband.
I don't know if I'd book Poseidon again, though, but this is because they cater more to snorkellers. Not that I have anything against snorkellers or snorkelling (which I love) but they obviously have to pick dive sites based on how snorkellable it is. Pure dive, the dive-only vessel costs a small fortune, but might be worth it if we can take our time and go dive sites that are good for diving, not snorkelling.