I'm not a Canon user, but Nikon makes two very similar lenses. Canon however has multiple sensor sizes, so if you are using a 1.5X sensor the 100mm lens is like a 150mm lens, 1.6X is 160mm, so not a great lens for anything but fish heads, and macro. The 60mm will allow tight full fish shots assuming you have the light and vis to pull it off.
Both lenses work very well topside, in fact, that is what they were designed for, and both work as normal lenses topside and UW, but with close focusing capabilities to 1:1.
If you want to see examples, I'm sure there are some on Canon's website in the *lens* area. They may not be UW, but that hardly matters much.
My Gallery here likely has some stuff shot with a Nikon 60mm macro lens, but I'd have to go look. It's been a while since I have updated it.