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.
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.
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.
