It really depends on your location. If you are in Florida, you would have a hard time because most commercial works just go to dive shop or underwater equipment stores.
It also depends on what kind of cinematographer are you. Such as if you are event shooter, marketing shooter etc. If you're marketing shooter, then chances is you would do a lot of shooting in the pool, which you can get into quite quickly. If you're event shooter, then yes you do need more dive experience. The trend of bride/groom underwater photo/video is not taking off. So I don't know what kind of outlet would you have. Does any of your existing client show interest in UW video? The drone is outpace the Underwater scene so far.