An advanced weapon doesn't merely rest upon the virtues of it's payload or the delivery of it's payload. Any weapon that can knock down it's competition by the virtue of being able to look first, shoot first, outrun, outgun, outmanouevre and outperform it's competition, or any combination of the above, is enough to be sufficiently advanced. Take any contempory weapons system and justify the ability for it's performance parameters to be expanded or improved, and it will be sufficient enough to be a game-changer in underwater warfare.

Indeed, UUVs are looking like the way of the future, as they don't have to worry so much about pressurised hulls and the meat-bags inside. This is handy when you want to outmanouvre a conventional sub, because if a sub goes to depth to avoid detection from sonar, these happy little chaps could be lying in wait to commit more souls to Davy Jones's locker. They can loiter in waterways for greater periods of time, years if need be. They would have passive detection, with highly complex computers to tell the difference between a school of fish, whales or pressurised hulls. They would be like the old sea-mines of now and the last century, only when they activate, they slowly move towards the threat and do nasty things to it. Mines with minds.

But for more sexier ideas, take ideas from nature. Materials that offer structural strength at depth, but are soft enough to be malleable to form precise hydrodyamic control surfaces, can form the basis of subsurface fighter subs that can patrol under the water at a great rate of knots. If you want to be even more advanced, have these fighters able to deploy from the sky like a seabird, strike underwater facilities, and then burst forth once again to RTB.

But yeah, as for actual weapons systems, there isn't much that is new under the sun. Rockets, torpedos, all good stuff. All that they need are expansions to their performance envelopes.