A light mounted on your housing is definitely the way to go, but I'd urge you to avoid the narrow "pencil" beam spotting lights some people use for this purpose. A spotting light doubling as your dive light needs to have a wide enough beam coverage to let you see what's in front of you and to the sides.
(Spotting lights "built in" to some strobes or attached to the strobe to help aim it can be useful for that, but won't be very satisfactory as a substitute dive light.
Personally, moreover, I think separate narrow pencil beam lights are very problematic anyway. Unless you are willing and able to shoot everything at exactly the same distance, you'll have to re-aim the spotting light for each subject and each time the camera (or the subject) moves closer or further away. Life is hard enough.
Of course, a wide-angle beam bright enough to be useful as a general dive light and to facilitate your camera's autofocus needs to be a fairly powerful light.
I use a Mini Hartenberger mounted on top of the housing. This light is available with an optional wide-angle reflector that can be switched with the standard beam reflector, and has multiple power settings that can be dialled from the rear. My light is mounted on a ULCS ball fitting bolted to the handle. This isn't the cheapest light you can buy, but my night dives have gone a lot better ever since I got mine.
If you plan to use a light mounted on your camera/housing as a dive light, do make sure you also carry a separate back-up safety light that's securely attached to you. This is a good application for a good LED light that has no bulb to burn out and can provide up to 200 hours or more burn time. (I don't much like the light that LED lights put out, but I think they're great as safety back-up lights.)
Frogfish