Thank you everyone. After shooting this weekend I agree that a strobe is the way to go, so I will go that direction. I got a few reasonable shots but not what I am looking for.
The lens is amazing but despite having 4000 lumens of light I had some focus issues. Many photos ended up out of focus (very close but not quite right) even at f22 and 1/160. In particular, if I set the camera to P (program/auto), A or S priority I can control the shot. However, in full manual all shots are virtually unusable and extremely dark (almost black). I was set at ISO 400. This was true even if I set the manual settings at what the settings were in one of the priority settings. I tried this above water and got the same results. Its almost as if the camera was compensating for low light in every mode but manual. Any idea why that would happen?