More serious problems are the time it would take to extend and retract the buoy, speed limitations while the buoy is being deployed, and noise mitgation on the buoy and cable.
Right now, submarines carry disposable communication buoys that can be deployed from depth and sent a pre-recorded message via satellite. Submarines can be alerted to a pending message while at depth through Extremely-Low-Frequency radio communication that can penetrate water, at which point the submarine can move to periscope depth and communicate via satellite without actually surfacing.
Long cables are strength limited. At some point you have to put floats here and there so it doesn't break under it's own weight
And then there is the parasitic drag. Long cables have large surface area.