Sonobuoys would give you eyes, potentially deep into enemy territory. They could be used to spy on enemy Outposts, or placed offsides of a "travel lane" between Outposts to monitor enemy traffic coming and going.

This is intended to go along with the balancing idea another player posted,specifically the limiting the extent of additional sonar added by each subsequent Princess to plus 50% of an Outpost's original sonar range. Sonobuoys would be a Hire option where you get three of them per Hire (the Assassins and Saboteurs come in pairs, but are more critical). Plus, since by Hiring them, you are forgoing a different Hire that would help your numbers (pop cap, production, combat effectiveness, etc), getting three would not be overpowered.

The way to use them is to click and drag a Sub to a spot in the water where you want a Sonobuoy. Just like the Pirate automatically loads up whenever you target another Sub, the Sonobuoy would, as well, when you are deploying it. The Sub will then take the Sonobuoy and drop it at the designated spot (Sub then disappears) where it will set up a Sonar signal that is half the diameter of a standard Outpost's Sonar (so, the size of a standard Sentry range).

If you want to move it to another Outpost (yours or an ally's) the Sonobuoy would do so like all other Specialists. Anyone wishing to grab a deployed Sonobuoy would have to target it with a Pirate (both the owner and opponents would have to use this method). However, if an opponent grabs it, it becomes a Prisoner.

Interesting concept - however, it has to be ensured that no other drillers or specialists could be on the same sub as a sonobuoy.
However, enemy pirates can use this to quickly move their pirates around, which might not be the best idea.