I have the 1DMk2. The 1DsMk2 didn't make it in time.
I liked the Sigma 150mm. The only problem being its refusal to go beyond f/22 which is something I'm trying not to do anyway. With the 1.25 crop, I prefer the 150mm to the 180mm. It's easier to shoot and is 2/3 stop faster for autofocus (which I had to use). It's also smaller and uses the same 72mm diopters as the 180mm. I imagine I'll like it with the 1Ds as well. It takes a 1.4x teleconverter better than the 180mm for the smaller stuff. I could more than fill the frame with the 150mm shooting pygmies.
As far as the Seacam, I did not like the 45 degree finder for macro. The 180 was better. I'm going to try the sport finder since I suspect I'll prefer it and the bouyancy is so much better. I don't like the Seacam handles. They're slippery and too contoured. You can't apply leverage when you need to. The focus knob is for the birds, but that will be fixed with the macro multiport that wasn't quite available. The housing itself is wonderful and with a few changes it will be perfect. I still hate autofocus for macro even though Canon does it better than Nikon.
I used the new Fisheye modeling light and it's very good. I blew a bulb in it though after about 90 minutes. It's nice and neutral underwater and puts out great light. The mounting arrangement is first class as well though the on/off switch is hard to use. This light will be an excellent, though expensive, choice. I'd like to see a narrower reflector.
I brought an Inon ring flash but haven't tried it yet. It's not an especially good match for the Seacam housing. It offers a +3 achromat wet diopter though. 
I tried the Macromate closeup lens with the Canon 100mm and did not like it. Too hard to use, too heavy, too hard to carry uw, too expensive and too easy to scratch. I didn't find the quality to be as good as the 150 straight up and the magnification (claimed 2:1) just isn't true. No way I could achieve that with my camera. I could get that with the 150mm or 180mm though. There's no substitute for the right lens.
The Woody's diopters work fine with the Seacam ports. I used the 100mm but the 110mm is probably easier to fit. I used a Canon 500D (+2) with the 150/180 and optionally the wet diopter. I also used the wet diopter with the 100mm without an internal. I'll probably use a mild internal diopter with the 100mm in the future.