I've seen a lot of pictures on the internet even taken at 1/160 sec having a lot brighter backgrounds than my pictures. That's why I started this topic.
Philip there are quite a few factors other than camera/strobe settings that will affect how much ambient light there is to capture. Some of these factors: depth (you seem to be diving quite deep), amount of sunshine above (is the sky clear?), position of the sun, direction you're shooting relative to the sun, etc. This is how someone is getting brighter backgrounds with a faster shutter. Try shooting up and into the sun (don't necessarily need to frame the sun in your shot).
Hope this help.