There are plenty of posts here recommending operators, so it’s worth looking back a few pages.
Without doubt the best trips are the overnight boats that visit the Cod Hole and Ribbon Reef areas. These are the areas far north and take a bit of sailing to. Some operators fly you up one way and motor back the other, so you get more dive time. E.g. Mike Ball and Spirit of Freedom. These are the best 2 in my opinion. Taka motors both ways, and is a little cheaper because of this.
They run 4 day cruises (leave afternoon Monday or Tuesday and return Thursday or Friday afternoon) to the Cod Hole. If you also want to see the Coral Sea (shark feeds and excellent visibility (30m / 100 ft plus) then do the extra day.
If these are a little pricey then there are some excellent trips out of Cairns that offer diving on the Outer Reef areas.
Pro Dive who run Scubapro are very good. Great crew and good dive spots. They depart every day apart from Tuesdays and unlike Ocean Quest and Kangaroo Explorer (who stay on the Reef and have daily transfer vessels go out) they depart and return with the same passengers.
The trips like Pro Dive, Ocean Qust etc are great trips for beginners and cater for students, snorkellers and beginer divers as well as haveing some more advanced sites (deeper etc), so you will be in good company and not be diving out of your comfort zone.
Here is a link on our site that give s a few details of the live aboards mentioned above.
Cairns Live Aboard Over Night Dive Trips To The Outer Barrier Reef, Cod Hole And Coral Sea.
Also worth noting that its peak season now and most of the boats fill up very quickly (few months in advance) so its worth looking at availability soon, and dont leave it until the last minute.
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Cairns Discount Dive And Snorkel Trips. Live Aboard And Day Trips To The Great Barrier Reef