A couple of thoughts:
If you are shooting your on-camera strobe in manual (just enough to trigger the S&S, you will need to dial in the power on the S&S. You didn't mention how your in-camera flash was set.
I shoot one of two options on my Oly E-PL1:
First option: Camera on Manual set to ISO200, 1/125, f/8 to f/11, flash set to fill-in flash and the S&S on DS-TTL. This allows the S&S to determine how much light it needs to pump out.
Second option and my preferred option: Camera on Manual set to ISO200, 1/125, f/8 to f/11 with flash set to 1/64 and the S&S on manual (manual 1 or 2 depending on camera). This allows the photographer to determine how much light you need by using the light level control dial on the S&S dialing up or down depending on what the picture review / histogram shows.
You can use faster or slower shutter speed to darken or lighten the background (shutter controls ambient light). Similarly, you can change the f-stop to allow more/less light in from the strobe (aperture controls light from flash).
I also make sure I am in sync by taking a photo in a mirror to make sure the strobe is firing before getting on the boat. I've ended up shooting video because I forgot to pop-up my camera flash.
Good luck!