Not to mention still in the realm of science-fiction. Unless the space-programme is
vastly more advanced in this universe than it is in ours, and I suspect
@theJMPer would have made (even a throwaway) mention to that effect were it so.
...
...
Why not just build a Dyson Sphere and evacuate the whole human race into it, while you're at it!? Do you have
any idea how many
billion cubic kilometres of water you're talking about filling with mines? Even if that many sea-mines could even be
produced — and I suspect that providing fields of the requisite densities would require more steel and explosives than have ever been produced
in the history of the world! — simply
laying the things would take
millennia!
Not to mention the problems with maintaining the minefields against ocean currents, Abyssal minesweeping efforts, weather and tectonic activity,
or the fact that mines can break loose from their tethers and wander into your 'safe corridors' with trivial ease, or, or, or, or....