One important detail about the supercavitating torpedoes is that they travel faster than the speed of sound in the medium. If you're relying on sound for detection of threats, then you'll be dead before you can deploy countermeasures.

EM scatters very quickly in dense media (in a very wavelength-dependent fashion). Sunlight, for example, is pretty much gone in the upper hundred meters of the ocean. I wouldn't count on being able to deliver any real amount of energy beyond a few tens of meters in seawater.

Straight electrical is also unlikely to work because it will flow towards the nearest ground. In the case of conducting seawater, that will be in all directions.

Any mass without an onboard propulsion source is unlikely to do well in a high-density environment like water. There's just too much mass to push out of the way that will eat into the energy budget. This problem does suggest either stealthy things like fish-analogs with onboard explosives or very fast things such as rockets that boost themselves along the way.