I shoot with a Nikon D7000. The Canon lenses are pretty similar. The 60 mm macro is a great lens. For true macro, you have to get really close, which is usually not practical. You either hit the reef or you scare away the subject. The lens does well for close ups and small to medium fish. You can use a subsea 5 diopter magnifier to achieve more mag. It is a wet lens and works as a flip up.
The other choice is the 105 macro. It is more specialized. You have the stand off range to shoot macro. Also it has great bokah. The downside is narrow depth of field. You have the reach with this lens to be able to shoot small skittish fish and not have to get close enough to spook them. It is also good for small, skittish fish: blended, jawfish, damsel fish, butterfly fish, file fish and so on. With a wet diopter, you can get super macro but it is tricky to get and hold the focus for that.
Both lenses have great optics and are prett fast at 2.8.
For initial use, I would recommend the 60 with a subsea.
