What I am going to suggest may seem fairly obvious, so if you have already tried it, then please take my suggestions with a grain of salt. Also, keep in mind that my suggestions will be pretty generic since I have no idea what gear you are using.
First, set up your gear as you would have it when you dive. (In the housing with the strobes hooked up.) It really doesn't matter if you have the lens port on or not.
Step 1. Establish a "Control Image": Set the camera to Program mode. Set the ISO to what you would typically use and set the metering to center weighted. (Do not change the ISO or the Metering throughout the test.) Point the camera at something (I use a little toy dinosaur, for my tests but anything will work as long as you use the same thing for each test.) Take a picture.
Did the strobes fire?
What were your exposure data (f Stop, Shutter Speed & ISO)? Write these down.
How was your exposure? (Over exposed? Under Exposed? or About right?) If your camera has the ability to show a histogram, that is better than simply looking at the screen because you could have the brightness on the screen turned up or down.
Step 2. Try Shutter Priority: Set the camera to Shutter Priority and set the Shutter Speed and ISO to the same values as the result from the image you took when it was in Program Mode. Take as close as you can to the same image as you did for the control. Take a picture.
Did the strobes fire?
What was your f Stop? Write it down.
Were these settings the same as they were in Program mode or were they different?
How was your exposure? (Over exposed? Under Exposed? or About right?) If your camera has the ability to show a histogram, that is better than simply looking at the screen because you could have the brightness on the screen turned up or down.
How did this exposure compare to the one from Program Mode? (Better, worse, or about the same)
Step 3. Try Aperture Priority: Set the camera to Aperture Priority and set the f Stop and ISO to the same value as the result from the image you took when it was in Program Mode. Take as close as you can to the same image as you did for the control. Take a picture.
Did the strobes fire?
What was your Shutter Spd? Write it down.
Were these settings the same as they were in Program mode or were they different?
How was your exposure? (Over exposed? Under Exposed? or About right?) If your camera has the ability to show a histogram, that is better than simply looking at the screen because you could have the brightness on the screen turned up or down.
How did this exposure compare to the one from Program Mode? (Better, worse or about the same)
Step 4. Compare the three images: Take a look at the three images that you have shot. They should all be very similarly exposed, but there might be some subtle differences. Select the one that you feel gave you the best exposure and note the settings for this image.
Step 5. Shoot the image in Manual Mode: Set the camera to manual mode and set the Shutter Speed, Aperture and ISO to the same settings for the previous image that gave you the best results from the previous test shots. Take the picture. If your camera is functioning properly, the image that you capture on this shot
should give the same results as the shot which you used as the basis for it. If you still get a black screen, then there may be a larger problem than what this test will show.
The key to this test is to set a control image (the Program Mode image) and to then leave everything the same (ISO, Metering etc) and only change
one variable at a time.
I hope this helps to troubleshoot your problem.
