Hello Paul,
I like the image you posted. Good exposure, vertical composition is correct for your primary subject and it has depth. In terms of advice for capturing better wide angle imagery, my first suggestion is to gain a better understanding of your camera system.
The meter in the Motor Marine II EX is practically useless. If you are determined to continue using the camera for the foreseeable future, I highly recommend that you purchase a used (experienced) Sekonic Marine Meter II and attach it either to the base of the tray or one of the strobe arms. The meter is easy to use and reliable. Since you can normally find a used one for approximately $100 US, I would not go to the added expense of buying a digital meter.
Next, if you are not already diligent about keeping the 16mm lens and camera lens surfaces free of bubbles and debris, its time to get paranoid and become a clean freak.
Bubbles and other nasties can accumulate on the surfaces and wreck havoc with sharpness.
Thirdly, shoot when the sun is at its highest point if possible (10:00 am to 2:00 pm on average). This will allow you to use higher numbered f-stops to achieve greater depth of field. Also, shoot up or at worst straight ahead. Shooting up allows you to separate your subjects from the water column and improving the available light.
Fourthly, be conscious at all times of your strobe positioning. Never, never, never simply light up the water column, especially if your subject is more than 5 feet away or positioned where only one strobe can illuminate it without crossing your camera to subject path (either reposition the other strobe or turn it off). Try not to allow your strobe to light the blue water between your camera and the subject. If you can concentrate on only allowing the inner beams of your strobes to paint the subject with light, you will keep backscatter to a minimum. I suggest pointing your strobes straight ahead and positioning them out to each side of the camera about 30" for horizontal shots. For verticals, positioning is more tricky. Again, you do not want to simply light up blue water. It may mean that both strobes are on the subject dominate side of the camera. Or, you may turn one strobe off.
The last two items on the list are silhouettes and film. Turn your strobes off for silhouettes even if you are extremely excited and do not think you have time. And, in terms of film, Fuji Provia 100F and Velvia are good choices. Of course, there are other quality options. But, when you start getting good results with a brand, try to stick with it and get to know its limitations. Scala 200, a b/w slide film, is also something I keep on hand when shooting wrecks, large creatures and certain schools.
I hope there is something you can use in the above advice. I apologize if you find any of it too basic. However, not knowing your skill level, I thought I would just write off the top of my head as if you were one of my students.
Take Care,
Scott