if you have a pirate at an outpost you can launch a sub from that outpost to any other player's sub by dragging from your outpost to that sub. and you can't target one of your own subs.
OK ! I wasn't sure because the rulebook states "Sub carrying Pirate can target another sub.", not mentioning if it can be one of my subs. There's no other sub in my game right now so I can't test it but I'll do as soon as I can!