It was a topic before to make DIY optical cables. There were suggestions to simply use optical fibers as cheap options. There are many dealer selling it per meter.
To make the length variable, the fibers can be curled. Previously I tried to use a hair drier to form the fibers but I've just found the hot water approach better. I gave a detailed description on how to curl optical fibers on my blog:
I haven't tried it underwater but it should work well.