We went to Australia at the end of May to fulfill our dream of snorkeling the GBR (we have snorkeled many times in many other places). We wanted to see the Outer Reef. My spouse gets seasick, so it seemed the Coral Princess would not be a good idea (actually, it turned out that the patch worked very well and maybe we could have done that. The downside of the patch was severe withdrawal symptoms when removed; still, worth it.) I investigated Lizard Island, but Cyclone Ita took care of that. I then decided PD would be the best place to take day trips from, and I think I was right, if for no other reason than it's a nicer place to be for 6 days than Cairns. I had looked for information comparing different trips without too much luck, so I hope this helps.
Note: All 3 boats have excellent crews, excellent equipment, and similar food and drink.
Sailaway: We arrived in PD from the U.S. on a Tuesday and took this boat on Wednesday because it is a shorter trip than the others. It is also a catamaran, so it's a smoother ride. It goes to the Low Isles, which is not the Outer Reef. There were two snorkels, one from the island and one from the boat. The one from the island was pleasant: one sea turtle, many fish, nice coral. The one from the boat was short with not as much to see. It was a good activity to get back to snorkeling when we were still recovering from the long journey, but it wasn't as good as our best experiences other places.
Poseidon: We went on the Poseidon on the last day it was family owned. The next day it became part of the Quicksilver company, so I don't know what has changed because of this. This boat has divers and snorkelers, with dedicated crew for each. It goes to the Agincourt Reef. The 90 minute trip out wasn't too rough, but it was boring. There were supposed to be 3 stops. The first was quite beautiful. It was cut short because of a medical emergency and the necessity to medevac someone. This also necessitated cancelling the second stop. We did have more time, though, at the second location, Reef 4, which was wonderful because it was more beautiful than we had hoped the GBR would be. The trip back was quite choppy and boring.
Wavelength: This boat takes only snorkelers. The 90 minute trip out was choppy, but it was smoother returning. This aspect of the trips are boring, but there's no alternative. The 3 snorkels on this trip were at Opal Reef and they were the best of the 3 days. The third snorkel was in an area called Fishbowl, and I don't have words to describe how exquisite it was. The variety of coral, the colors, the shapes, the fish.....it was like an alternative reality. We did not want to leave and the experience will stay with us for the rest of our lives.
I think I've covered everything, but I'd be happy to answer any questions. Happy snorkeling.