I shoot Sony (a6300, recently upgraded to a6700), but your Canon 7D shouldn't be materially different. Place the strobe above your camera, facing directly forward. Put the camera in M mode, and dial the settings to your ambient conditions - I find f/11, 1/125s, ISO 400 to be a good starting point, with the strobe(s) at quarter power. Take a test shot of a rock or coral or whatever at your expected shooting distance, making sure to get at least some open water in the shot, and evaluate it - if the background water is too dark, reduce shutter speed, then if it's getting too slow (I don't like going below 1/60s, maybe 1/40s for static subjects) open the aperture a bit and/or increase ISO. If the foreground illuminated by the strobe is too bright, reduce strobe power, if it's too dark - increase it. If you modify aperture or ISO, adjust strobe power to compensate, i.e. if the foreground looks right, but background water is dark, and you're raising ISO from 400 to 800, dial the strobe a stop down at the same time. If you're getting backscatter in the top of your shot, raise the strobe higher, further away from the camera.
Keep in mind that a single Z-240 is not going to illuminate a whole lot, so you'll have to get very close to your subjects. A wide-angle lens is highly advantageous - if you don't already have it, look into getting a Tokina 10-17mm fisheye plus the appropriate port and gear.
