The depth of field issue: When using a macro lens your depth of field is very narrow so its best to shoot anything closeup with a higher aperture ie f16-22, shooting at F29 is not going to give you the best results from that lens. Using 400 iso is still ok, dont go any higher if you can help it. USe the Av setting to start, set it at f16, the camera will decide the shutter speed, use a tripod because your shutter speed will be low and it will blur. Also use the self timer to eliminate any movement that you might cause by pressing the shutter yourself. Once you are all setup, bring the lens as close to the glass as possible and switch to manual focus, using the ring at the front of the camera to fine focus on the minute details on a particular coral, depth of field from the f16 - 22 will take care of the foreground and background. You have one of the best Macro lenses Canon makes so take advantage of it.
One last thing this is only good if you want real closeup macro shots of a subject that isnt moving, like sessile inverts and corals. You would be better off using auto for the moment if you want to shoot fish or anything with a lot of movement.