What's the fastest way to gain Echoes in Sunless Sea with Zubmariner?
tl;dr
go to Nook with Wine and grind Outlandish Artifacts from "Take from the weak".
You must bring some Wine (about 10 bottles; if you can easily deal with Terror due to e.g. Magician, you can aim a bit higher). Also, first raise your "Taste of Freedom" to 15 (by doing virtually anything you want there) to get maximum success chance here (75%).
Currently (15.X.2016') there's no downside to a failure*, and a success rewards you with an Outlandish Artifact (and also with Nodules, useful if you fancy going deeper), the only prerequisite is that you have to find Nook first**. With 75% success (15 ToF), you'll get about 12 OA for a single Wine bottle and +5 Terror. Full 10 runs took me about 5 RL minutes, and left me with about 10,000 E worth of OA for just 50 Terror and 10 Casks of Mushroom Wine (200 E if bought in London).
* well, getting Nook-ish was intended to be a downside by itself, but it happens regardless of the event's outcome chuckle
** of course you first have to have a Zubmarine, but that requires only a single visit to Cavendish (which is very easy to locate anyway) for every Captain after the first Zubmariner you have in your bloodline; locating Nook is thus IMO the tricky part here.
If you consider it a cheat (I don't), you can try other ways; after playing through all current Zubmariner content, I can say that most of the events gain you only pocket change or are one-shot. Sadly, while the trade opportunities in Zub ports nicely complement the current ones, in monetary terms they are IMO only barely better than the vanilla ones, if at all (no new cargoes have been introduced currently). If you don't want to farm Nook, you should probably stick with the "vanilla bunch", still profitable:
the well-known Surface route: have full crew and about 25 Fuel, fill the rest of your cargo bay with Coffee from Port Cavendish, hit Cumaean Canal, sell your Coffee in Vienna, haul cargo in Mediterranean until your crew drops to about half; helping the Anarchists a couple of times (6 max) and selling Recent News in Naples gives you various free stuff making startup trivial. If you don't want to see London in flames and supremacy: Anarchists gets to 6, simply report them to the authorities for additional bonus.
For extra E buy some (about 10 will suffice) Mirrorcatch boxes, fill them and sell the Sunlight to Blind Bruiser, until your Menaces: Y,B gets close to 140 (Sunlight trade gets too risky then IMO); watch out for The Revenue Men, though (having >=5 Favours with both Antiquarian and Admiralty gives you virtual immunity to them, though).
Hauling cargo gives you ~ +200 E for every ~2 crew members lost, when taking the price of food+fuel into account; each Sack of Coffee sold gives you about +40 E.
After your Menaces: Y,B get too high and Vienna no longer accepts Coffee (it's best to first sell exactly 59 Sacks, and then drop batches of 60 for best result) you can reuse the Boxes in Hunter's Keep "Visit the cellar" to get a couple E more of this enterprise. Since afterwards you're only left with the option of hauling cargo, you should probably switch to route "2" by then.
- Red Honey route: wait for SAY on Isle of Cats, buy one vial, rinse, repeat; after you spent all your E this way, go to London and sell (Brass Embassy is IMO the simplest way, as it comes with no risk) - you'll get about 400 E gain for every 600 E invested. You can smuggle it safely if you get rid of your SAY immediately after you get it on Quaker's Haven, and then go full-speed to London; still, even with 500 E paid in case of emergency, you can easily almost double your E this way.
If you don't want to be a smuggler or don't want to spend half an hour hunting parrots, and if you don't mind using a cheat-sheet or have a perfect memory, with a bit of luck you can use the route "3",
- Scintillack route: raise Pigmote Isle's Civilization to 10, buy as much Scintillack as you can there (for 55 E), and sell it in London (if you're a practical sort) for +15 E/unit or in Iron Republic (if you're the risky sort) for +19 E/unit, giving ~ 1-2k E for one trip with a ship with sufficiently big cargo hold.
note that you'll have to click absurd amount of times in Pigmote, since you can only buy one Scintillack unit per click sigh
If you don't want to have CTS or simply aren't lucky enough to get PI's Civilization to 10, you can always use route "4", and go to College:
- Mangrove Swamps route: have Terror as low as you can and buy as many Foxfire Candles as you can, and then go to Mangrove College's Wisp-Ways. Most of the end results there are very rare and highly valuable, including (but not limited to) Captivating Treasure, Searing Enigma and A Lump of Blue Scintillack.
There are of course other ways (Enigma/Surmise trade, Egg grinding in Elder Continent using Lampad-class Cutter with WE ARE CLAY and Rats, Zee-monster treasure-hunting etc.), but they ain't IMO nearly as fast or rewarding as those above.