Whether used for a touch of fill lighting, or to provide all of the light to illuminate a scene, understanding the proper use of strobes is essential to successful underwater photography.
Flash units can certainly play an important roll above water too.
Underwater this is less likely to be true, particularly in the often-dim waters of Monterey Bay. Lack of a strobe or the understanding for proper use places severe limits on the types of underwater photography possible. Several factors conspire to lead to the reliance on strobe lighting. Water, being far denser than air, acts like a light sponge. Even clear tropical water steals about half the intensity in the first 10 feet of water.