I would suggest starting with an F-stop in the 5 to 6 range and a shutter speed of 80. The real key is to understand what each setting does for you. For the most part, the F-shop and strobe level control the near ground exposure, if your close in subjects are over exposed, lower the strobe level or go to a larger number F-stop, the reverse if they are underexposed. The background, areas past where the strobe will effectively light, is controlled by shutter speed. The slower the speed the lighter blue the background will be. Depending on your subject and your skill, speeds lower than 1/60th are hard to keep still enough to prevent blurring the subject.
A good way to learn how to use the controls is to shoot outside in the evening, a little before sunset. Set up some object...I like a flower arrangement.. a few feet from your camera. Fill the frame with mostly the subject but have at least 25% of the photo as blue sky. Start with you camera set to F-5.6, speed at 100 and strobe at full power. Adjust the F-stop and strobe power to get a good exposure. This is a really good time to learn how to use a histogram if you camera has one, the cameras LCD will often not show an accurate depiction of the actual exposure of the shot....it will lie to you...the histogram will not, its your friend, learn to use it.. Note that the sky will remain pretty much the same color and lightness no matter what you set the F-stop or strobe to. Once you get the near field exposure set, play with the shutter speed. You will find that as you increase the speed, the sky gets darker and darker until it's black but the exposure of the near field items only change slightly. (This does need to be done in late afternoon to simulate the lower light conditions that are normal in diving). I have a series of photos that I took to demonstrate the effects of shutter speed, Fstop and strobe power. I will try to add them this evening.