Looking at your requirements, first off I'd say don't write off shore based. With two morning dives, an afternoon and a night dive, you're at your 4 dives/day.
That's the schedule I ran in Bonaire, but I could drive to the dive sites in the rented truck, and dive whenever I was ready. My experience with shore-based diving elsewhere is that you get to the sites in a small boat, and the dive shops run either one 2-tank trip or two 1-tank trips a day, and that's it. It's completely not clear to me which dive operations run more dives than that, at least from the web pages. The descriptions of the all-included packages often state that you get just 2 dives a day, plus unlimited dives of the house reef.
I received my Fisheye 500DX from Reef Photo today. It's bigger than I remember the HG20DX being. A good bit bigger. It's almost like having a third strobe, in terms of bulk.
It seemed pretty nice, and I didn't have the problem HarryM did with his copy - fairly clearly he got a defective light. It didn't seem quite as bright as I was expecting, so being nerdy, I decided to test. I did my best to replicate the test conditions described in the Backscatter Focus Light Shootout. I shot a white surface at maximum output at a distance of 12" and noted the metering at 100 ISO. I assume the article images were shot at 100 ISO - the photos are missing their EXIF data, so I can't say for certain, and the article doesn't say.
As you can see, beam width is very similar. The article specifies a white balance of 3200K, which makes the 500DX's beam appear very blue, even though it's white to the eye. I make the beam at about 20" in diameter at 12", where the article says the HG20DX is 18". However, the 500DX has a very definite 5" diameter hot spot, where the HG20DX's beam is very uniform.
I'm less certain of the metering. I did try and duplicate the article, but even a slight difference in distance from the camera will make a definite difference in exposure. I measured 1/60th at f/10 and 100 ISO. 2 stops wider implies the 500DX only puts out 1/4 the light of the HG20DX, despite drawing about half the power. I can see why the 1000DX exists, though using 8 AA's instead of an expensive proprietary battery seems like a real advantage. I can easily stock a spare set of NiMH AA's for a lot less money than the $150 price of an extra lithium battery.
I also tried the light at minimum power, and metered at f/4 and 400 ISO. That's 4 stops, or about 6% of maximum brightness. Fisheye claims 150 minutes with 2000mAh batteries at full power, so 40 hours at minimum power? I suspect less, but I'm not really interested in testing that.
What matters in practice? I've only done a few dives with a real focus light, instead of relying on my dive buddy illuminating the target with her primary dive light. Beam width definitely matters, since you can't count on the light being precisely aimed at your target at macro distances, and your subject may not be center-frame at longer distances. The light doesn't have to be strobe-bright, it's not supposed to be a source of light in your photo, just enough for autofocus to work reliably.
At macro distances, beam brightness is almost a disadvantage, since it might add a hotspot to your photo. For longer distances, brightness matters, but a bluer LED beam almost certainly penetrates water better than a ~3600K incandescent light like the HG20DX. I think a really fair comparison would be underwater, and distances of 3' or so.
I am using the Eneloop or other low internal discharge batteries and during a trip to the Philippines, I was getting more than 60 minutes at full power and much more than 80 minutes at 2/3 power.
I have a bunch of those Eneloops - they're 2000mAh at 400ma draw. I'm not really set up to test draw over 500 ma, but I found a site testing them at 1.2a, and he got about 1850 mAh at that level (NiMH do hold up to high draw well). So 1.850 x 1.2 x 4 = 8.4 watt-hours gave you 60+ minutes, which implies the actual load is about 8 watts at full power.
It'd definitely nice to hear from someone who has used all 3 lights we're discussing. I'm halfway wishing that I'd gone with the I-torch light, given your recommendation, but I suspect the differences are minor. Especially since I'm lugging around a DSLR housing, so the difference in weight is small compared to the total weight of my rig.
Yes, you'd expect twice the burn time with half the power draw, unless the light's power management wasted energy. If anything, it should go a bit longer, because battery capacity is better at low current draws. We know the I-Torch's draw is about 1.9 amps at full power, which is really, really high. I strongly doubt you'll get 90 minutes of burn time at full power. Even a Sanyo 2700 is likely to give you maybe 70 minutes at that kind of draw - stated battery capacities assume a 10 hour discharge. Of course, maybe the stated 9 watt (3 bulbs x 3) draw is high, but given the claims of brightness, that doesn't seem all that likely.
However, it's worth noting that you really don't need the light on all the time during a daylight dive. Unless you're shooting crevices or the underside of a wreck, ambient light usually gives you enough for easy focus. It's night dives where focus light burn time really matters, and I suspect the I Torch will probably last long enough for intermittent daylight use and a full night dive.