You're right about submarine cables running DC along the shielding/armor to power the optical amplifiers. However it's worth pointing out that there are so-called "repeater-less" systems that do use optical delivery of pump power to the amplifiers (typically they combine this with Raman amplification). Those systems can deliver high capacity communication (not sure where the record stands at but 100s of Gb/s to low Tb/s) over >500 km without any electrical connection (you still need power at the receiver though).

These are typically used for short submarine connections to e.g. connect an island. As it is much cheaper than running a full repeatered system.