Submarines captured my imagination first when watching Voyage to the Bottom of the Sea and Stingray in the 60s. Today’s subs can circumnavigate the globe, without having to surface, because they have nuclear power plants on board.
Even these high-powered monsters would benefit, however, from not having a giant conning tower sticking out of the deck. This allows the crew to see further when travelling on the surface but adds greatly to the drag when moving underwater and makes the boat highly visible when surfaced.
Today’s invention is a virtual conning tower. The submarine’s hull would be a single, streamlined torpedo shape with minimal protrusions. Instead of a tower, a small flock of UAVs would be released from the casing of the submerged vessel, break the surface and then fly at variable altitude and perhaps at a small distance from the mother ship.
These would be steerable from below, enabling advanced reconnaissance via onboard cameras (submarines can communicate subsea using radio transmissions at around 80Hz). The UAVs could be made effectively undetectable by radar and provide all the benefits, in terms of damage tolerance, of a swarm of aerial robots (at most, these might be seen as a flock of seabirds).
It might even be possible to have a tubular hull shift ballast so as to stand nearly on end in the water -acting as a temporarily high conning tower. This would also allow the diameter of the hull to reduced, cutting drag significantly.