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First of all, some cameras do not include a built-in flash, at all. This is often the case of higher end SLR and Mirrorless cameras.
In this case, to achieve the best colors and exposure underwater, external (outside the camera housing) flashes are needed. In order to fire those flashes, a small on-camera flash or trigger is needed to relay the message from your camera to the strobes that it is time to “flash”. If you’re interested in learning more about this, we have many resources on the topic, such as this one.
However, some cameras, including most compact cameras, include a built-in flash.
So, one question we often get is, “is the internal flash good enough?”
In short: not really. Most cameras’ built in flashes are very weak for underwater photography. Even with an effective range of around 12-18 feet above water, this usually translates to no more than 2-3 feet underwater.
And, many housings for underwater cameras are designed to channel the built-in flash to external fiber optic cables, meaning the flash is mostly obstructed for traditional use.
If your housing does offer an unobstructed flash window, the built-in flash may work for closeup shots with an open aperture, but it will not be effective in almost any other type of shot.
Don’t get me wrong, it’s better than no flash at all, but for getting good results , you need an external strobe.
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