[DoingIsLearning]

Optics would need line of sight. The earth's curvature puts a very hard limit to how far you could communicate. The accepted standard in underwater is VLF, but as the name implies you are only capable of very low data rates.

[ac29]

The name implies only the frequency, which doesnt have anything to do with data rates. The VLF band is tiny though - only 27kHz. The small bandwidth is the main reason it would be slow.

[DoingIsLearning]

You're correct from a DSP standpoint but the point remains VLF is very slow.

[trasz]

Curvature doesn’t affect line of sight between the sub and a satellite.

[DoingIsLearning]

True but if your emitter is not underwater like the receiver then you will have to account for air/water refraction not sure how that would work in the real world. Also I suspect it would be fairly easy for a rogue state to infer submarine locations from emitter beam orientation in low orbit space.