In 'flash' modes of the HF-1 you would have a 'focus' light mode (the 1500 lumen light you mention, but three levels, low/med/high) you could use, but it's not the same (not as bright) as the 'video' mode - that's actually a good thing on a night dive: at night I typically leave it on 'low' or 'med' so as not to blind my buddies.
In video mode there is no flash, just the light. I tried video once, so far, on a night dive (with domed diffusers) and it was bright... like Mack-truck-on-a-highway underwater bright, lol.