@Chris Ross and
@tursiops, I guess I wasn't clear or maybe we're saying the same thing. First, regarding shooting your strobe in a room above water--I assume we're talking shooting in Manual and RAW. My point was that I don't see how one can replicate how dark or light the underwater conditions are by turning off the room lights or dimming them. As you know, the light or lack thereof underwater can change very dramatically in 10 feet or so, and so if you're at 100 feet it may be a lot darker (depending on whether is is clear or cloudy up top or not) than it is at 50 feet. And light is not light at depth; it is totally different in terms of strobe use. So I think it would be hard to replicate those variances in a dark(ened) room, and I've never heard anyone (eg, from backscatter or bluewaterphoto) recommend that approach. Everyone I've spoken with--and all the Internet sites I've researched--suggest getting shutter and aperture where you want and then experimenting with the strobes' power output.
Second, and very much relatedly, as you know, at depth, one uses shutter speed to "dial a blue" and to achieve the color one wants in the background, since the strobes will light the foreground irrespective of shutter speed. So the faster the shutter the darker blue the background. As noted, the shutter speed will not affect the strobes, since they fire far faster than the shutter ever could, so the amount of light provided by the strobes is almost entirely a function of the power setting you have them on. Now, let's say that you like the way the background of your shot looks shutter-wise, and you're happy with your aperture, but when you fire the strobes your foreground is too dark or too hot. Above water, without strobes, if you like your speed and aperture settings, then maybe you adjust your ISO or even your exposure compensation. But neither of those work well underwater: as noted, adjusting your shutter will only affect the BACKGROUND and not the foreground where the strobes are firing. Similarly, increasing ISO or EV will increase light receptiveness for the ENTIRE photo, meaning both the background and the foreground will be more (or less, as the case may be) exposed. But again, that's not what you want, we've agreed you like the way the background (shutter speed) looks and the aperture. Moreover, because the strobes are so powerful, increasing the ISO or EV even a little bit usually results in the foreground getting blown out--strobes are really really sensitive to increased ISO.
So for all these reasons, you want to first dial in your shutter and aperture at the lowest ISO you can achieve that makes the background look good/blue, etc., and then use the strobe power levels to get the foreground exposure the way you like it.
Here are a few examples where I used this technique:
https://flic.kr/s/aHsmFrVnhn
I hope this makes sense. Thanks.
Ps. One additional note: the YS-D2s are so powerful I have yet to encounter a wide-angle shot I could not get with them on or near full power, see, e.g., the very wide shot of the fans in the link.
