Not an expert at all and moreover, I have never been interested in underwater photography, just plain HD video, but I have faced the same dilemma: should I invest in a digital camera with video capability, just in case I find the shot of a lifetime? I eventually dumped this idea as irrelevant. The reason I am chipping in is that I was recently on a boat of UW photographers, one of only two videographers out of maybe 20 people. The first main difference you notice is that their lights are different: they use strobes, not flooding lights. To take advantage of the strobes, you need fast shutter, which a camcorder does not really provide you with (more so now, like in my CX550V, but it's not coupled with a TTL output for strobes, so you cannot take advantage of strobes with a camcorer).So if you want to do video AND photography, you are pretty much limited to a digital camera. Large camera housing are probably stable enough (due to their bulkiness) and just as expensive as camcorders. BUT, it seems that camera drain their batteries quite fast and camera housing are maybe a miracle of technology, but to me they look way to complex, with way too many controls... I love the simplicity of the L&M housing where after a few dives you don't even look at the controls anymore.
Then their is indeed the resolution issue and zooming capabilities while shooting. AND memory capacity. Camcorders come with plenty (and NEED plenty), while that's not necessarily the case with digital camera. You can buy memory cards, but then 64 GB cards are in the many $100...
In summary, my 2 cts are: like photographers decide whether they want to shoot macro or wide-angle before they jump in the water, you should probably make your mind on whether you want to shoot video or stills before you buy anything. Then buy the best for that purpose. It seems hard to do both well with a single rig.