That is one way you can also passively tap a fiber without breaking it, I am not sure if it could be done deep undersea, I suppose it is possible with the right set up. Basically how it works is you have to open up all the cladding and get to the bare fiber (not always that easy) but assuming you can do that, then you can take another fiber and put it right next to, parallel with the main one and then apply a little heat, the proximity of the tapping fiber will pick up a small signal. It is called a fused biconical splitter, it is possible to do non-intrusively.