When you’ve advanced a few levels in The Witcher 3 you’ll be introduced to the Isle of Skellige, where the main quest will move its focus to.
Since Skellige is an archipelago of islands, it’s very easy to take a boat and sail between locations. When you do this you’ll start to discover hidden Smuggler’s Caches, floating in the sea around the islands. Maps to these are also sold by some merchants. These are a great source of income. Most Smuggler’s Caches contain three chests of loot, although a few have two or four chests.
Unfortunately monsters patrol many parts of the sea, so you’re likely to run into them as you explore.
The most common monster when exploring the seas of Skellige are Sirens – flying female creatures that swoop down on you if you come too close to them. Sirens can also dive straight from the air into the water, and will come after you if you’re submerged. So it’s worth learning how to deal with Sirens before setting out to farm Smuggler’s Caches.
If you’re at the helm of a boat and want to fight Sirens: immediately drop the helm, take your silver sword out, and stand on the boat. Wait until the Sirens fly within range and strike at them when they dive to attack you. Aard and Igni can also knock them out of the air if you time it right.
If you’re sailing and a Siren grabs the side of your boat: immediately go into combat mode and start attacking it. If you leave a Siren attached to a boat it will eventually damage it, so you want to clear them away as quickly as possible. Remember that if a boat receives a certain amount of damage it will sink. And that is pretty embarrassing if you’re miles away from shore. But it happens to everyone at least once…
Watch out not to collect too much stuff when you’re out sailing. You might be able to loot five or six Smuggler’s Caches in one trip. If you do get over-encumbered, sail to the nearest port and whistle for Roach when you get to shore.
One last thing to note is that the contents of Smuggler’s Caches are randomised, based on your level. If you wait until later on in the game, before looting the caches, then the items they give out to you will be higher level and therefore worth more money to sell. It’s also easier to out-swim monsters, and take damage from their hits, when you’re higher-level, so that’s another reason why you might want to wait until later. My advice is to loot a few caches fairly early, but leave the majority until later in the game, and when you go out on sailing runs: plan a route with your boat so that you can easily locate three or four caches in a row.
You want to be collecting and selling lots of Great Sun Armors, and ancient silver swords, that are worth thousands to the right buyer.