Gudge's/eskasi's statements are what I discovered, as well, after my Nikon buddies (like SCOTTY G!!!!) got me all excited about TC's - teleconverters won't work with the 60 mm EF-S lens. Insert expletive here.
To get greater than 1:1, I use either (1) my 60 mm EF-S lens or 100 mm EF lens with a flip-on Macro Mate (ridiculously expensive, but I do love that thing!! Woody's diopter is another way to go and may not give you much different results) for tiny things I can get very close to, like nudibranchs or (2) the 100 mm EF lens (Canon) with with either a screw-on a diopter (I just started a thread about this on this board a few weeks ago - I have been using a +3 the 2 times I've dove it), which allows me to get shots of skittish things, like gobies/blennies.
In my (limited) experience....Combo #1 can get lovely results, but it can be very frustrating in surge - you have to be able to get close and stay close. Combo #2 gives you a bit more working room for things that are likely to take off, but it does not provide the same magnification as combo 1.
Good luck!
Allison