I've used my cheap Toslink cable on a few dive trips, no problem past 50m depth anyway. Their seem to be two main types, the ones with 1mm plastic fiber optic which is the one which is fine. A slightly more expensive one uses a much finer glass fiber cable and this has a more open structure which would fill with water, not what you want.
Cool. I made two different cords. Both audio cables were relatively inexpensive. One was $12 for 13' and the other was $16 for 10'.
The $12 cord is thicker and 'soft' with a rubbery outer casing (maybe 5 mm - kinda like my Ikelite electrical sync cord but a bit thinner) around a thin 1MM optical fiber. I had to strip the outer casing to reveal the fiber prior to sliding into the adapter.
The $16 cord was more like the stock Sea & Sea cable with a harder plastic coating around the optical core. This cord fit right into the adpater. I used the hair dryer trick to put a curl into this one.
I'll try them both out next week and report back ... I think I prefer the heavier rubbery one so far. I put a set of cable ties on the arms to guide and hold the cord and these work great keeping the heavier cord in place while the curly thin one is harder to thread through the cable ties.