Ditto what James said re the Sigma 150 and the 180's. I don't have a 5D, but use the bulkier 1Ds MkII, which is also full frame, so lens issues are the same.
I personally have gone back and forth about getting a 180, and keep putting it off because I find that the 100mm, the 150mm, and -- most recently -- the 100mm with a Kenko 2X TC (making it effectively a 200mm) do the job. Plus, you can add a wet diopter (depending on housing) to use in those cases where you want a little more magnification DURING a dive. Depending on where you're diving, and how clear the water is, you can use some of these lenses as fish lenses as well as macro. For example, the 100mm is a GREAT lens that is like that (it lets you stay JUST far enough away from fish not to spook them).
In short, if I were buying a 5D right now, for macro my wish list would be (all these are 1:1 macro lenses):
- Sigma 50mm (great all-purpose lens when not sure if you'll be shooting macro or schools of fish, and the primary lens I use on night dives)
- Canon 100mm (the one I use the MOST)
- Kenko 2X TC for use with 100mm for super macro
- Sigma 150mm
- Wet diopter (e.g., Woody's, or Seacam "Sea Two" if you buy a Seacam housing)
Now...what I DON'T know is how to manually focus the 100mm lens with 2XTC, at least not in a Seacam housing. James -- any ideas? 