Ah, but you see...
Did Mr Doubilet say the diving was good? If I recall he didn't!!!
Well, I can't speak from personal experience on Cozumel, so I can't compare with Belize. Where did you go in Belize?
As for PNG, well you need to look at the "land tours" link on the PNG section of my web site to see that this place has more than enough to offer you. The largest tracts of untouched rainforest in SE Asia, the best bird watching in the world, the most amazing cultural melting pot of people (706 individual languages - not dialects, languages - from a population of 4.5 million), volcanos, incredible scenery, brilliant trekking, tons of WWII stuff, surfing (like wow, man), white water rafting. You name it, it's got it!
As for the Red Sea, well the desert does pall after a while, and the interesting stuff, like the ancient Egyptian monuments (with a random chance of being gunned down by a lunatic!) are not accessible from the Red Sea unless you fly.
As for the diving in Oz, having worked on the GBR & in the Coral Sea for 18 months in the late 80s I can assure you that it in no way compares to the diving in PNG. I have been 7 times, and the shortest trip I have done there is 3 weeks. I have only scratched the surface.
Til the next time!
Jim