I live in New Jersey and ambient light even at 70-80' is a joke. The meter doesn't even move. I have worked on using long shutter speeds to enhance the photo. By long, I am taking 1/4 to 1/2 second. I have found that I can shoot at 1/4 second without too much trouble. Going to 1/2 second becomes difficult. To overcome this, you can place your housing on the wreck to keep it stable or even bring a monopod with you. An expensive monopod works because it's life span in salt water won't be very long. One of the drawbacks of longer shutter speeds is fish or diver movement. If you don't use rear shutter synch on your camera, the flash will fire and then the fish or diver will move before the lens closes. You will get a glow in front of the subject as they blurr the image. If you use rear shutter synch, the flash will fire at the end of the period that the lens is open and the blurring will occur behind the subject - giving a feeeling of movement. You can see photos taken at longer shutter speeds on my website in the New Jersey Shipwreck section or in the Artificial Reef section. In the pictures of the shipwreck, Lana Carol, here:
Herb Segars Photography : Lana Carol, all the photos were taken at either 1/8th or 1/10th of a second with apertures around f8 using (2) Ikelite DS-125 strobes with an ISO of 200. Most were taken with a Nikon D100 in a Nexus housing.