Advanced aquatic warfare

[rdanhenry]

Nanomachines that eat through enemy subs, but not your own. Flood the sea with them.

[Freax]

supercaviation has been mentioned...
Being stealthy is easier in water, because water blocks radar and heat signatures pretty well, and you can't look that far.
Sound is a problem.
You need of course sound-absorbing materials to avoid Sonar reflections, but some kind of clever foam or rubber could solve that.
Your own noises travel far, but this also could work for you, because you can hide in front of the background noise from all over the world if you are silent enough.
For high-distance-attacks that have time (like against a big ship with a known future location, a fixed station, ...) a weapon-system could drift with the current or even hang onto a large animal and only cover "the last mile" with a short, high-velocity run. When the enemy can detect it, it already is to close to be stopped.
Working in a swarm would probably also help to avoid counter-measures, because an enemy could tell that something is there, but the exact locations would be hard to determine when there are 100 tiny noise sources.
There have been several attempts to train animals to attack enemys, or even to make them controllable through electrodes in the brain. Mastering that also would constitute as advanced (not so much from the moral, but from the technological point of view).