Drones that sound like, act like, or maybe look like whales, might be the answer. If the adversary can't tell if its a whale or drone, likely to force them to waste a lot of resources and/or ammunition.
It's a compelling picture, but it seems like it would be rather harmful for real whales, and if chances are it's a drone rather than a whale, several actors might choose to fire regardless and find out later.
Not that whales are the only option to mimic, but whatever is chosen had better be common enough to make the drones rare by comparison, and stay that way.
Jellyfish might be a nice option when conditions permit...
A coded sonic pulse could have exceptionally long range. Sure your enemies would detect it, about half a second before they detect the drones.
A more practical concern is simply temperature and how long the drone's power supply can survive in the cold ocean.