Yes, the 180mm Macro is slow to focus, and yes it's hard to get a good shot out of it, but it's fun to use.
Without a manual focus gear, most situations call for lock and rock focusing. Even then I am finding it hard not to pull it out of focus when taking the shot.
I now have possession of a 72mm 500D diopter to try out. But I've run out of Subal extension rings for now, so I'm yet to try it underwater. Out of the housing I have been playing with combinations of the 180mm + 500D + 25mm ext tube. If I can dive a site with zero surge, stop my heart from beating, and hold my breath, I might be able to hold focus. 
In very clear water, I think the 180mm has it over the 100mm for sneaking up on critters, but I'd have to say that's about where the advantage ends. I'll see what the diopter does for me, but I'm expecting it to become harder to get a sharp shot.
BTW, my favourite lens is still the Canon 60mm Macro.
The photos below were taken at Tulamben, Bali this year with the 180mm Macro alone on the 7D.
1/250 f/13 ISO400
1/250 f/7.1 ISO100
1/250 f/7.1 ISO200
