While many photographers on land achieve their best images using only natural light and never touch a flash, the same is not true most of the time underwater. Beneath the surface, color is lost very quickly. By about 15 feet, most of your reds will be gone, and oranges and yellows are absorbed shortly after that. If you wish to bring back the beautiful colors of the underwater world—a source of artificial light (or a filter) is a crucial tool.
Underwater strobes can bring that color back and provide a tremendous amount of power in a very short burst; they can be hundreds of times more powerful than a continuous beam light. For this reason, they are greatly preferable for freezing fast action over using high shutter speeds. Syncing with strobes can happen in one of two ways: electronically via the camera’s hot shoe or via a fiber optic link from the camera’s on-board flash.
Generally syncing I mean speaking, syncing electronically is more reliable and does not require the camera’s flash to recycle which is definitely a benefit for firing rapidly. However, it has a few downsides, namely that it generally does not support TTL flash metering (you should be shooting in manual anyway), and that the cables are sensitive to water and are a failure point of the system. Fiber optic cables, on the other hand, are not affected by water and provide fully automatic exposure.