I used to have a D70 and did quite a lot of shots with Ike housing, a single DS-125 and 60mm. I typically used 'M', 1/125 @ f8 if it was a fish portrait, say like an Angelfish or Grouper. Using a larger-numbered F-stop is certainly possible, but you won't get a nice blur for the background. On tiny creatures like a Pedersen Shrimp or a Jawfish, I would use 1/125 @ f11 or f16. Keep your strobe set at TTL, Adjust your TTL + or - exposure on the housing.
Of course, this is just a "suggested" place to start. Sometimes you have to twiddle with things to get what you want. For example, here's a pic of a 2-inch long trunkfish who was moving around quite quckly, so I had to use 1/250 @ f 3.3 and I'm pretty happy with the shot. It probably would have been a bit better at f 5.6, but good enough. Definitely keep the camera in Manual for macro stuff, it's easier to change the exposure by using the buttons on the back of the housing for the strobes. On some shots, I would have the TTL 'minus' setting on the back of the housing down as far as it would go.
For that matter, if you are busy doing stuff and don't have a lot of time to mess with your camera, set it on Manual, 1/125, f11 at ISO 200 with the strobe set to TTL and the TTL adjuster on the back of the housing to -1. To start out with, adjust your strobe arms where the strobe is directly over the center of your housing, perhaps 3 inches above it, then adjust it to point 1 to 2 feet in front of your port as needed. This should get you some good shots.
Also, I use a Woody's Diopter for getting that 60mm to 2x magnification for the really small stuff like Nudibranches; check it out, for $90 it's an excellent addition to your gear and can be taken on and off during the dive as needed.
Good Luck! Remember to post some pics when you get back.
