Two-staged desing with engines and fuel at first and sensor only at second should work - if you can't launch(or rather throw out) directly at point where you want it.
So just design your buoy - sensors/reactor only. Research it. If you can drop it directly, nothing more needed. Else, make another missile design, with hi-effecient engine and enought fuel. And put 1 buoy what you designed earlier as an second stage. Should work.
Missile with engine(/s) will never become an buoy it will be. . . emm. . . Missile.