Went this year, 4-day Mike Ball liveaboard that included a puddle jumper to Lizard Island to meet the boat. There is one other liveaboard operator, and they both alternate 4 day liveaboard to Coral Sea and the other to Cod Hole and closer in places, both operate out of Cairns. We went on latter 4 day trip that starts on Lizard Island and ends in Cairns. The flight over the reef to meet the boat off Lizard Island was a huge highlight. It was awesome to see the reef patches from the air. It was def dive, eat, repeat several times a day, and sleep. We did see bleached coral. It was a bucket list thing for sure, but we have enjoyed seeing healthier coral in other places in SE Asia and the Caribbean.
Met a woman who worked as a marine biologist in the GBR for past 27 years, and she confirmed that it is nothing like it was. We met her on a whale watching snorkel in Moorea (and we saw lots of humpbacks up close and personal in the water there), and we also saw over 30 spotted eagle rays swimming over us, over and over again (surreal) on a dive in Bora Bora, something etched in my memory forever.
Still glad we saw the GBR when we did, but I think the flight over is the most memorable to understand the scale of the reef area (almost the size of Germany). A pinnacle we dove on the GBR had the healthiest coral. The problem, confirmed by the marine biologist, is that the coral in the northern areas has been badly damaged/bleached by several back-to-back warm water bleaching events and a cyclone last year. She said there is healthier coral in the southern area of the reef, but no liveaboards operate there. My advice would be to look to dive in the southern GBR area if possible. If you do either of the GBR liveaboards we found running out of Cairns, you will have fun and meet some cool people from all over the world and get in lots of diving. Frankly, we were disappointed in the reef life though, with the exception of a few pinnacles we stopped at. Still something to see and we did have a good time!
If you are able, try to fit in New Zealand while in the area. The South Island is just beautiful!