First of all, I think your picture is lit quite good with a single strobe.
By saying "some photos lose depth of field and look rather flat" i presume you mean a more 2D look of the composition.
Regardless of the number of strobes you are using you ll be having a strobe lit foreground and an available light illuminated background.(as the dark reef section in your photo)
I believe having a little bit more background in your photo could give more depth to it. This of course depends on the topography of the dive site. I first thought that you could have approached a bit closer and get a bit down and shoot with a slightly upward angle but than i realised that if you have done that in this particular photo you might have totally lost the reef on the background. If this is the situation you could try to have water surface texture or the dive boat etc on the background.
If what i have written above is irrelevant and you meant the way you lit the foreground part, by moving the strobe from central position to side you could have more shadows and texture on the foreground. I believe you can do this easier with an off-center single strobe than double strobes.(in the latter you should have done inward lighting for that effect )