Ed I was going to suggest you try the camera without an external strobe, but then I remebered you wanted magazine cover pictures. For that you will need an external strobe because the editors will not accept anything else (since they like all other UW photographers are brainwashed).
I am a voice in the wilderness on the wonders of the internal flash. Several reasons for this, the big ones are, size of the camera, a microscopic increase in range and lack of issues with backscatter. I love being a diver first and not having the hassle of my camera gear being as big as my diving gear. My camera fits in a BC pocket and safe doing a giant stride. The power of the internal is more than enough to reach out to almost all subjects, if its outside that range the picture will suck anyway.
Now backscatter the great bain of UW photographers. My regular dive spots are close to the crap you dive in. I find that most pictures are down (or level) with the subject, under those conditions there is no backscatter. If I have to shoot up into the water column I will get backscatter. Here's where post dive processing comes in to remove the backscatter, a simple and fast technique in photoshop with the clone tool.
There are numerious examples in my web page or galleries. I use a Sony DSC-P5 and INTERNAL strobe.