Indeed each camera will have the flash sync speed, at least this is true for those that have the mechanical shutter and if you set the speed higher than that, the flash partially will be obstructed by the second curtain traveling before first curtain got fully open while flash firing. Some of the flash units have so called high sync flash which is essentially constant illumination throughout the opening of the shutter, I am not sure which underwater strobes have this function. I do have S&S YS-01 but haven't seen this function in its manual. Said that normally i would not setup such a high speed (1/2000) underwater as I won't have sufficient exposure even with wide open aperture (unless being very shallow on a bright sunny day and cranking iso to something like 3200) so assuming you are using something like 1/125 or less, you should be within the sync speed and at this point it will be only the aperture that will control flash illumination. It looks like you are on the right track using shutter speed of 1/80 - 1/125 with not too high iso (200 seems like a good starting point for COZ during the day). You are also correct in your statement that as long as your shutter is lower than the flash sync speed, it's value will not affect the flash illumination only the b/g exposure in combination with aperture and iso. Aperture value will impact the flash output, the higher the value the less flash range. I would suggest to play with the aperture (for proper ambient exposure) and flash power (for foreground illumination) keeping iso and shutter at constant values. By limiting yourself to only these two variables it will be easier to see how they impact your images. Have fun!
