Sunpak PF20XD in optical slave mode held up and to the left. The SX230 flash was set for Manual, Low to trigger the Sunpak. A finger lanyard helped me hold the SX230 firmly in my right hand while the left held the Sunpak.
The Sunpak was shooting through about 3x4 inches of diffuser and this may have helped some as the flash was fairly close. Likely didn't help much. The manual flash on the camera would have filled only a bit.
The SX230 focus assist light worked well enough for the shot. It took a couple tries, though. It was dark, dark outside Bahia Drake on the Osa Peninsula in Costa Rica.