can you adjust the speedlight's flash interval like you would adjust the exposure interval on the camera? Some speedlights will let you cut the light up to 2.5 full stops.
You could also set your speed light for high speed, adjust your ISO to 400 or even up to 1600, then open the aperture and use a faster shutter speed to get your desired effect.
F10 gives you clarity through out your depth of field, but when shooting fish usually you are shooting close to one focal plane.
Good luck