Hi, just a quick reply, I know the more experienced guys like Acroporas will give you a better answer.
I use a 5d, Ike housing and dual DS-125 strobes. Like you, I am very experienced on land, but UW was a whole 'nother thing!
For macro, I use the Canon 100mm macro lense. Camera settings are: M mode (1/125th at f22 - or f16 if I'm only using one strobe like recently), iso 100 (two strobes) or 400 (one strobe) [i am still fiddling with this - I think I can get to f22 with one strobe at 400, but I keep forgetting to do the test to see just how much DOF I can get at full strobe power, so I can tell if I need to use the higher iso or not], Auto focus, custom function to "strobe fires", strobe exposure to evaluative.
When your strobes are your primary lightsource and you have good TTL, M actually works the best. On land, you tend to want to use Av, with aperature fixed and shutter speed floating - but underwater you have that extra variable (strobe power output), so you can use that as the floating variable and leave the other three - f stop, aperture, iso - fixed (as long as you have enough juice in the strobe to cover the range of likely exposures needed). You might have to do a little fiddling to figure out where your strobe maxes out. If you want alot of depth of field, as you will for macro, you could do the tst that I've been forgetting to do on every dive by setting the strobe to Full instead of TTL (and I think you would have to switch the housing controls to Manual too), setting f22 (or whatever aperature you want to use for max DOF) and seeing how low you can go on iso before you run out of light. As a starting point though, in murky dark southern california water, you can use f16, iso 400, 1/125 and a single DS-125 will have plenty of power for your average nudibranch shot.
With the macro lense, I preset Tv mode for 1/125 for when I want to take that rare non-strobe photo (got some harbor seals that way recently, was glad I had it there). 2 steps to switch to non-strobe work: Primary strobe to "off", dial to "Tv" instead of "M". If you're on 400 iso, it will be ok.
I have recently started shooting macro with the Ike circuitry set to "minus 1", since "proper" exposure was giving me some blown highlights, and esp blowing out my red channel alot on certain shots (i.e., spanish shawls - orange stuff.) You have to remember to reset for each dive though.
The Ike TTL circuitry should be on TTL (if you want TTL of course, which you do, since it works so well), and the switch on the bottom of the housing set to the correct one for your strobes. My housing has a choice of DS-50 or DS-125, yours the same? Strobes themselves on "TTL".
For wide angle, I'm not that experienced (total noob actually, only used this on my Cocos trip), but I got good results with the strobes by using M at 1/125 or so, and metering the blue and correting aperture pretty much constantly. (You never know when that sailfish is gonna swim by - I got lucky, my settings were close when he did.) Iso on 100 for most of the time.
I have found 1/125 is about as slow as I can go - there is enough ambient light to cause blur in the background even if I'm using the strobes for primary light source.
I always shoot RAW. Saved a bunch of shots that way.
Good luck! It's loads of fun, my 5d loves to go diving!
Taxgeek/Susan
(I have some galleries with exif info you could study at www.pbase.com/taxgeek.)
