Hi Gang,
I went out to the Flower Gardens this weekend to put the finishing touches on my macro setup for my upcoming 3 week Bali trip. I shot
I was very happy with the Sigma 150mm + Canon 500D diopter. It's incredibly sharp. I shot all of these w/ the MkII and the Seacam housing with magnified viewfinder.
Here are a few samples, from 1:2 macro to almost 2:1
1:1 Orange Cup Coral:

Greater that 1:1 Feather Duster - this one is slightly cropped:

1:2 Frogfish:

More photos here:
http://www.reefpix.org/gallery/album122
Cheers
James