Craig and Spen, I need to "expand" a bit on the technique I mentioned for your particular photos, as this is what I use for "bride & groom" RAW images, which your excellent frogfish photos fall into, i.e. we need to extract (especially) shadow detail (the groom's tux, the fish) and the highlight detail (the bridal gown lace, the sand); so this technique applies to these particular frogfish scenes.
First and foremost, you have to watch the histogram in the upper-right corner: The ultimate goal in the RAW converter is to expand the histogram curve as wide as you can without clipping; so that when you get to the Shadows/Highlights tool you have enough "meat" in the dark areas to grab on to. [Of course, this means you'll be converting to, and then manipulating in, 16 bit/color depth.]
Step 1, while holding down the ALT key, click & hold the Exposure slider: The preview screen will turn black. Then, as you move the slider to the right, you'll start to see blown pixels in the color they are blown, i.e. you see which color is blocking up first. Usually, you'll see the yellows block up first; but since it's just sand (as opposed to bridal gown lace), you can tolerate a small amount of this... But, watch the histogram. This step is a bit counter-intuitive, as this is where you open up the shadows;
Step 2, while holding down the ALT key, click & hold the Shadows slider: The preview screen will turn white. Generally, if I have a scene involving extracting shadow detail, I set this to zero. Again, this step (actually, the "shadows" name) is counter-intuitive, as it is here where you open up the highlights. Since we have sand, (again, as opposed to lace), we can safely leave this at zero;
Step 3 is to get your color balance (not just white balance) close -- It can be fine-tuned much better in the main program. Here's a "quick & dirty" trick I use: I look at what the camera "As Shot" tagged the WB at, noting the temperature and tint; then in the pop-up I select what Photoshop thinks it should be -- The "Auto" setting, again, noting the values. Sometimes they are very close, sometimes they diverge widely. If they are close, I pick which one I like. But, if they differ widely (such as 4800, -24 and 6000, +8), I then split the difference (in this case, 5400, -8) as a starting point, then tweak it. You'll also notice that the pop-up changes from Auto (or As Shot) to Custom;
Step 4 is to touch up the brightness & contrast, keeping an eye on the histogram: Note how the curve shifts shape as well as position as you move the sliders;
Step 5 is a repeat of steps 1 & 2 if necessary to touch-up the histogram: Remember, the goal is to make it as wide as possible in the RAW conversion step, to give the main program as much data as possible to work with, but without going overboard -- You'll extract even more shadow detail later on in the main program;
Step 6 is, if you are using PS/CS is to set the sharpening slider to zero (under the Detail tab); as you'll want to sharpen just the Luminosity channel in the main program. If you are using CS2 or later, go ahead and apply the sharpening here, as Thomas Knoll completely rewrote ACR's sharpening module to do just that. 
Now, hit OK to convert and send the 16 bit file to the main program, where you'll first save it as a .PSD file. (If you don't first save coming out of the converter, the memory doesn't get freed up -- Internal PS stuff)
Next, you'll go to the Shadows/Highlights tool and start out with the settings I gave above, then fine tune accordingly, all the while keeping an eye on the Histogram in the Info palette;
Finally, you'll fine tune the color and apply sharpening. Save as a 16 bit .PSD file; then convert to 8 bit and Save As... a JPEG for web, yada yada yada...
Hope this helps!
Dan
[by the way, I'm still using ACR 2.4 (part of Photoshop 8, err, CS), since I use Win2k/Server on my workstation, and PS/CS3 needs Win2k3 or XP/SP2.]
(Cut)
My defaults for ACR are different than most. I zero out vibrance, saturation, contrast, brightness, blacks, fill light, and recovery. I then set my tone curve to linear and save that as the default. When I convert a raw file, I first set white balance, then exposure, then recovery, then blacks. Any remaining adjustments, if any, will be related to contrast and dynamic range. The image will look initially very flat until you make some adjustments. Don't be discouraged.
I prefer linear tone curves as a start because they are required when shooting test charts to get accurate results. It's not necessary for your camera to be accurate, of course, but I like having a reliable starting point and I feel calibrating to a standard produces better underwater images for me. That's entirely subjective. The downside is that images look bad until they are adjusted.
As others have said, your details are in the darker parts of your image and so correct exposure is important. Beyond that, your choice of contrast settings will have a great deal of effect on the way the frogfish looks. If you are using ACR, there is no need to use curves or shadow/highlight controls as Dan suggests, since ACR allows you to specify custom tone curves directly inside it. It's always best to get as much of the basic image right in the raw converter as you can.
Gary, I'm sipping on a Green Apple Bite flavor Smirnoff Ice... 
Now, it's off to watch them wreck some more at Texas on ESPN2! 
After reading this thread...I need an aspirin.
