The name of the game in Sea of Thieves (apart from 'Sea of Thieves') is piracy. And what do pirates want, apart from adequate vitamin C to ward off scurvy? That's right: gold! Filthy lucre! Vast hordes of riches with which to buy fancy lookin' coats and suchlike. Here are a few tips to getting rich quick, or at least quicker than you would without them, in Rare's new piratical PvP adventure.
Always check for extra treasure chests
Never leave an island as soon as you've got what your voyage demands from it. The bigger an island is the more likely it is to have bonus chests tucked away somewhere, additional to the animals or treasure or skeletons you've been sent to steal/capture/kill. If you've got the time and there are no rival sails cresting the waves on the horizon, spend a few minutes having a quick scooch around. Check caves and high spots and tucked away places, but don't underestimate the possibility of there being quite a high value chest just… out in the open. Like it's no big deal.
Always check for stuff that isn't treasure chests
When you first start playing the game you might not be told that you can sell a bunch of stuff to the Merchants Alliance rep in the outposts, over and above doing the voyages they commission you for. When you're out and about adventuring, keep a sharp eye out for barrels of gunpowder, or crates of silks, spices, sugar and tea. Like extra treasure, you can often find them in caves, around jetties, or abandoned or ruined houses on the larger islands. Depending on your rep with the Merchants and the rarity of what you've found, you can get a couple of hundred to a few hundred gold for packaged goods.
Similar to above, it's worth bringing in an animal every now and then to supplement your regular income. A lot of the time you can find an empty pig cage or a chicken crate abandoned on islands you're exploring, and it's good to have some on hand so you can do a bit of ad hoc animal capture. Animals have rarity, just like chests, and there's not much point in transporting and selling a pink piggy or a white hen, but a golden or red speckled chicken, or a black or striped pig will both supplement your income from an island for just a little extra effort. Both animals and packaged goods are worth far less when you sell them on the fly versus when they're a requirement of a voyage from the Alliance, but it's still worth hoovering them up if you have spare crew.
Look out for circling birds
The sea be a dangerous mistress, says I, and many an unlucky cove has lost their ship to the watery depths. But their loss is your gain! While you're off on a voyage, keep a look out with your trusty spyglass for gulls circling over the open ocean. They mark a shipwreck, which you can swim through and collect, usually, an astonishing amount of booty for very minimal effort. You should definitely leave someone on board your ship to keep an eye out for other ships, though, because shipwrecks are such easy money that they can attract a bunch of other pirates and become, in effect, difficult money. The key with looting shipwrecks is to be very, very fast.
Do a fort
Assaults on forts are tough, but if you've got the manpower you can make literally thousands. You know those giant skull clouds floating in the sky that you can see for miles around in every direction? That marks a fort you can raid. You'll need a fully stocked galleon and four crew (though three can probably manage it, in a pinch). Once you take out the cannon towers on the side of the island you approach, leave at least one crew member on the fort at all times to keep the ship afloat, provide covering fire, and one eye out for other pirates approaching the fort. The rest of the crew have to survive waves and waves of increasingly powerful skeletons and a final boss who'll drop a key for the fort's treasure room. Which will have loads and loads of treasure. Seriously, loads. So much. You'll be rich. I spaffed all mine on a golden cutlass.
Run voyages alone if you don't have loads of time
You don't (currently) make any more or less gold for a voyage you conduct alone versus with a crew, but your gold to time ratio does go up if you run fast and dirty in a sloop by yourself or with a two person crew. There's way less faffing around, so you can just throw down a voyage, hightail it to an island and then scuttle back to the outpost to make some quick cash. This is especially good if you've not got a lot of time to play. Chances are that if you're on a tight timescale, with a full crew there'll be so much arsing about that you'll have to leave before the treasure gets turned in.
Join a Galleon crew half-way through their game when they've already got an unbelievable amount of loot on board, thus reaping the profits whilst doing very little work
This one is particularly efficient.