For macro photography, strobes are the sole source of light. With longer focal lengths and shooting with small apertures (higher f/stops), very little, if any, ambient light gets to the sensor. The use of one or more strobes is what lights the subject.
One strobe can be enough to properly expose a macro shot, assuming the subject is small enough and you are close enough. However, properly exposed does not always mean the lighting is pleasing. Often times one strobe results in harsh shadows and flat images. This can be remedied by the addition of a second strobe acting as fill-light. Additionally, using two strobes allows for more creative options. Hence, dual strobes are highly recommended for anyone who is looking to seriously improve their underwater photography (macro or otherwise).
It's important to realize that each subject can be lit in many different ways, and often times the creativity of the shot is in the creativity of the lighting. A great example of this is backlighting. Back-lighting is useful to give subjects a sharp outline. Point the strobe almost straight toward the camera lens, but block it with the macro subject to achieve this effect. An optional second strobe can again be used as a light fill to give the subject some colour. Obviously, this is not always the right lighting technique as it creates a silhouette.