Last one for now.
The ol' Bait and Switch.
Nothing brings me more joy than this strategy. Have a slowly escalating arms race? An untrustworthy neighbor? Or just a sub carrying 500 drillers headed your way? This strategy is for you.
The Bait: One of your border outposts that you believe is most likely to be attacked (whether its a mine, bordering someone you're at war with, or just an attractive spot for an ever-growing opponent to expand to. I will typically help incentivize my opponent by making it clear that if they attacked it, they will win that attack. Don't overdo it, but it has to be apparent.
The switch: A few switches can be used here, but it typically involves racing either a double agent or martyr to your "bait" outpost that will arrive before your opponent gets there.
IMPORTANT: As you play more experienced players, the key to this strategy working is to have your "switch" specialist out of your opponent's sonar range. Only low level players will not realize they are being baited if they can see the switch prior to sending an attack. This is why I say you need to "race" a sub there as you will probably be a ways away depending on your opponent's visibility. Smugglers, Helsmen, and Admirals definitely help.
Which switch to use when:
One single sub carrying a large amount of drillers/ and or specialists: Double agent. As you are probably aware, if you have a double agent arrive at your outpost and then send directly at the incoming sub by itself, you will take control of all of your opponent's specialists and kill all his drillers. Pro move: If you have some movement bonus (pirate, admiral, or helsmen), wait to send the double agent until one tick before the attack occurs, and then just send a sub out after your double agent to reclaim him.
If you don't have a double agent, try an assassin. If your opponent is going to win based off their specialist involvement, wait until one tick before the attack occurs and send your assassin at them. Even if your assassin kills their specialists but then is captured by their drillers, you will easily be able to catch him with any kind of movement bonus as noted above.
Multiple subs within a tight vicinity: Martyr. Instead of sending one massive sub, some players now send a new sub every tick with the total drillers divided between them. (This counters the double agent defense). So here you will want to send the martyr as soon as it gets to your outpost, to hopefully save your own outpost and take out all subs within the blast radius.