The best way to target a sub (if possible in the first place), is to point to the sub's destination, then slowly move towards the sub's origin outpost until you see the time to impact has the (sub) text. You may have to wiggle your finger back and forth a bit to have the pirate latch on. Note that there is a glitch where you can lock on, but not catch up in time, resulting in the pirate arriving at the outpost and not the sub.
Reporting from the Bridge