something to mention... the 180 only provides working distance, it is still the same 1:1 magnification, it is L glass, might be faster focusing, and maybe a better image as a result... debate that forever. but GENERALLY should not produce an image that the 100mm cant also produce. albiet at a longer working distance. in all honesty, even if you have the greatest water clarity in the world, bringing your subject closer to the tank wall should help when possible.
I like the canon 100mm. the tamron is the stuff for nikons, because the nikon 105mm is like 900$ with VR and all that jazz. but if your going 100mm, may as well get the canon lens for the money.