I'm interested in this too for later in the year. Although my interest is very much on the snorkelling side. Also open to departures from Port Douglas.
If snorkelling is your priority I recommend Silverswift/Silversonic (and indeed I will redo the trip out of Cairns this July)
Pricing & Fares - Silverseries Scuba Dive & Snorkel
I have been on both both boats/trips and both were excellent.
You can book direct via its website or via agents.
Discount of various sizes can be had via autoclubs or others.
Note that it does not run everyday at present (which seems quite common for many Cairns Regional activities at present.) and so make sure you select a day that it operates on if booking online.
They are sister ships that operate out of Port Douglas or Cairns to different regions of the reef.
They are fast = more snorkelling time and also less time on the water in between which is where people can get sea sick, if they are likely too. Speaking of which I always take a sick sickness tablet prior to your trip as as it is not worth risking your trip, and I am someone who often charters smaller yachts for upto 14 days at a time.
On my first reef trip out of Cairns almost forty rears ago I looked around and noticed that there was a lot of crew. I asked a crew member why, and they gave me a knowing grin and said you'll see. Sure enough about 30 minutes later there were passengers barfing everywhere and the crew active at making sure they did not go overboard or make too much mess etc. Fortunately the boats these days tend to have a kinder motion. but I still recommend taking a tablet.
Anyway back on snorkelling.
The
Silverswift/Silversonic have roaming licenses and have a larger number of sites that they can visit. They
visit three outer reef locations and will choose 3 that best suit the weather and se conditions of the day.
You swim off the boat's swim platforms. They do not have pontoons, but this to me is a good thing.
The food is ok but the priority of these trips is the snorkelling/diving and not the food. So if you really want the big bang up buffet lunch experience go on one of the cruises to the pontoons. Better food, but worse snorkelling.
Over the decades I have gone on many snorkelling trips in the region from Mission Beach all the way up to one out of Cape Tribulation, as well as as far south as from Airlie Beach. For a good day out snorkelling, the Silver Series I would rate as the surest bet for good snorkelling. I still enjoyed the others though.
Book your tickets online for Silversonic Dive & Snorkel, Port Douglas: See 979 reviews, articles, and 314 photos of Silversonic Dive & Snorkel, ranked No.29 on Tripadvisor among 29 attractions in Port Douglas.
www.tripadvisor.com.au
For those with young children, some that include the inner reefs and sandy cays may be a better option.
To mix things up we are doing snorkelling trip that we have not done before, though I expect the actual snorkelling component to be tame. and so are doing it more as a rounded fun day out to a location that we have not been to be before.
Frankland Islands Reef Cruise - Exclusive Cairns Great Barrier Reef island day tour with river cruise, snorkelling, guided island walk & a beachside buffet lunch.
www.franklandislands.com.au
For my favourite non-snorkelling trip in the region I highly recommend.
Tully River Rafting Tours. Australia’s Best Rafting River. Allow around 11-12 hours for this one. Pick-ups from Cairns and Mission Beach.
www.ragingthunder.com.au
Tip: Do it out of Mission Beach as it makes for a much shorter day.
My wife and I are doing our tenth Tully River Rafting trip together for her 60th Birthday Present

There used to be two suppliers, but it looks like one took over the other.