Without a doubt the Canon EF 100mm f/2.8L Macro IS is my favorite macro lens for underwater work. It offers quick fast focusing and produces crystal-clear images with beautiful bokeh. It works well with both APS-C and full frame bodies and incorporates Canon’s new hybrid image stabilization, offering up to 4-stops of shake compensation.
Navanax inermis, EF 100mm f/2.8L Macro IS, Canon 5D Mark III, 1/160, f/10, ISO 160
I find it perfect for shooting subjects about three to four inches or smaller underwater. The 100mm focal length also makes it a wonderful lens for topside portraiture work. At a minimum focusing distance of 12” you will get 1:1 life-size magnification. This gives you enough room to light your subjects and a comfortable working distance not to scare off skittish fish.
A male Orangethroat Pikeblenny displaying his large dorsal fin to attract a female. EF 100mm f/2.8L Macro IS, Canon 5D Mark III, 1/125, f/10, ISO 160
If you need to get even closer and gain more magnification the Canon EF 100mm f/2.8L Macro IS works excellent with wet diopters such as the ReefNet SubSee +5 and +10 and the Nauticam SMC-1. Nauticam also makes the Multiplier-1 which can be added to the SMC-1 for extreme magnification!
Simnia with eggs, EF 100mm f/2.8L Macro IS, Nauticam SMC-1, Nauticam Multiplier-1, Canon 5D Mark III, 1/160, f/13, ISO 160
I use a focus gear with my Canon EF 100mm f/2.8L Macro IS USM. The full-time manual focus override allows me to fine tune focus at any time without the need to switch the lens out of auto focus. If your housing allows you to comfortably use back button focus, I would highly suggest you give it a try.
Skeleton Shrimp with Eggs, Anacapa, CA, EF 100mm f/2.8L Macro IS, Nauticam SMC-1, Subsee +5 Canon 5D Mark III, 1/160, f/11, ISO 160