I propose a solution. Invest in unmanned surface vehicles to carry duplicates of mail and require every piece of mail sent to sailors to have a duplicate of which randomly is selected the actual copy of mail to send to the real navy ship. Collect duplicates of the mail and send it on unmanned surface vehicles to sail around and pretend to be navy ships while gathering surveillance data.

Then even if someone thinks they have planted a tracker on a ship successfully they will still be second guessing themselves over whether they are tracking the real ship or not.