I was just looking into the topic recently, there is a decent graphic explaining how fiber is already used for submarine communication for quite some time.

Page 13 shows the diagram of an undersea fiber network separating into durable heavy and lightweight wires.

    Sea surface
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        |
        | near-surface armored cable
        |
     [Depressor]
          \
           \  fiber cable
            \  0.010 inch diameter
             \ up to 40 km long
              \
            [Float pack]
                 \
                  \ 50 m secondary cable
                   \
                 [Vehicle]

The sea is a very hard place. Heavy Chains break all the time.