I'm using a Nikon 200mm f/4 macro lens on a Nikon D610 DSLR. I posted about this lens here:
https://www.reef2reef.com/threads/new-macro-lens.437478/#post-5094821
While my 200mm is an amazing lens, it's not entirely practical for coral photography due to its size and focus distance. This lens needs to be at least 10" away from the subject to be in focus. This makes it really awkward for top-down photography and I had to custom order a really long top down portal to be able to get shots of any corals near the water surface, and I need to stand a on small ladder to be in position with my tank height.
You should be fine with the Canon 100mm. The key thing is to be sure you know how to use your camera in manual shooting mode, use a tripod, and a wireless shutter release if possible so you eliminate any vibration. I also shoot with a really small aperture to maximize depth of field.