Why not more fibers sub-sea? I would have guessed when you are going to the expense to lay fiber across an ocean, including a fair amount of dark fiber looks like a good investment.
Repeaters and equalizers. They go up to 8 fiber pairs, and are probably the most expensive single component for long cables. You need separate amplifiers/equalizers for each single fiber, so the cost strongly depends on the number of strands.
Since these repeaters are typically placed every 50-80km, there's a significant number of them in long systems and powering them becomes a challenge. These cables usually operate around 10kV DC at 1-2 amps, and I'm not sure how realistic it would be to go higher. The wire resistance losses are probably already more than 70% of the voltage budget, so you can't really use more current (voltage drop = I^2 R). Higher voltages cause other issues such as dielectric breakdown. Deep ocean cables are actually pretty thin, about as thick as a garden hose, and I doubt you could easily pass more than 15kV. You also have to account for ground voltage shift between the continents which may be as high as 1kV and depends on the weather.
There is a limit to the amount of power that can be pushed through the cable on each end, and each strand of fiber needs a number of Erbium-doped fiber amplifiers, which consume a lot of power. This isn't a problem on land, as you can add power at each regen site.
Since these repeaters are typically placed every 50-80km, there's a significant number of them in long systems and powering them becomes a challenge. These cables usually operate around 10kV DC at 1-2 amps, and I'm not sure how realistic it would be to go higher. The wire resistance losses are probably already more than 70% of the voltage budget, so you can't really use more current (voltage drop = I^2 R). Higher voltages cause other issues such as dielectric breakdown. Deep ocean cables are actually pretty thin, about as thick as a garden hose, and I doubt you could easily pass more than 15kV. You also have to account for ground voltage shift between the continents which may be as high as 1kV and depends on the weather.