An Skellig has now been fully explored, leaving Faroe as the only substantial landmass in Skellige to not know the tread of your boots. Before you head down south, though, there's an awful lot of treasure off the western coast of An Skellige, between here and Spikerogg. It would be a shame if all that poor, orphaned loot just floated out there forever without a master, wouldn't it? Time to remedy that and adopt some treasure, and clear some map markers and kill some beasties while you're at it.
Smugglers Cache (Levels 16 and 18)
Grab a boat and sail west of An Skellig to your first map marker, a Smuggler's Caches (what else?) guarded by some Drowned Dead (level eighteen). You'll find this event roughly lined up laterally with the "Yngvar's Fang" signpost. Another Smuggler's Cache event can be found to the west, beyond a hatchet-shaped island. Sail to the western side of this island and find the Smuggler's Cache event on the waters north of the island's peninsula, which itself trails to the south-west. Kill the Ekhidna (level sixteen) swimming around and loot the chests. There's also a Guarded Treasure event on the eastern end of this island, but since it's guarded by a Basilisk (level thirty-two) it can wait.
Hidden Treasure (Level 16)
South of the hatchet-shaped island you're near sail south to find another island, where you'll find a Hidden Treasure event along the south-western end of the island. Dive off your ship and kill around a half dozen Ekhidna (level sixteen), then make your way to the Hidden Treasure marker and dive underwater to find half a sunken ship. In the hold of the ship you'll find a chest, which can be looted to complete this event.
Smugglers Cache (Level 18)
Sail west and slightly north to find another Smuggler's Cache event, which is protected by a pack of Drowned Dead (level eighteen). Kill them and loot the chests, then board your ship and prepare for more adventures.
Guarded Treasure (Level 16)
From the last Smuggler's Cache event sail north-west to find a small island, along the western edge of which is a Guarded Treasure event. Exit your boat nearby and swim to shore, then fight off a flock of Ekhidna (level sixteen) before heading east across a grassy patch of elevated ground between the nearby beaches. Shortly you should find a ledge you can jump up onto, upon which is a chest, which is the object of this event.
Spoils of War (Level 18)
Sail north around the island and make your way north-east to discover a Spoils of War event. Leave your boat and dive to find a sunken ship, which has drawn the attention of Drowned Dead (level eighteen). Kill the necrophages, then descend to the ship and search along the north-western side of the ship to discover a chest.
Guarded Treasure(s) (Levels 16 and 18)
Swim north to reach a beach, then follow the southern coast of the island to the east to find a Guarded Treasure event. Kill a host of Drowned Dead (level eighteen), then search the beach to find two chests sitting in a watery depression. Not enough treasure? Of course it's not. Make you way to the eastern end of the island to find a small, hook-shaped bay around which numerous Ekhidna (level sixteen) fly. Kill the screeching beasts, then dive into the bay to find the treasure they were guarding in a small sunken boat. Still not enough treasure? Make your way back to your boat, then sail west from this island to find yet another Guarded Treasure event. This one appears to be in the middle of the water, but when you arrive you'll find that a small island upon which a few Drowned Dead (level eighteen) pace is the venue. Chop them up, doing the best you can in the confined bit of land you're on, then loot the chest resting near the island's one, small tree.
Smugglers Cache (Level 19)
West of here you'll find a cluster of islands, the largest of which is also the closest (eastern-most) island. Sail to the waters to the east of this island to find a Smuggler's Cache event between two smaller islands. The derelict ship and the flock of Sirines (level thirteen) and Ekhidna (level nineteen) should give the place away from a good distance. You know the drill, dive into the water, shoot the beasts dead with your crossbow, then loot the nearby treasure.
Don't discount the large, eastern-most island of this chain as mere eye-candy, however, as you can find a chest in a depression on the eastern side of the isle. Granted, you'll have to fight a number of Endrega Warriors (level eleven) and an Arachasae (level eleven) to get it. Additionally, on the southern end of the island you'll find an unmarked Bandit Camp, where several Bandits (levels fourteen and sixteen) are guarding a number of sacks and crates.
Guarded Treasure (Level 18)
Get back on your boat and sail west, between the two smaller islands west of the large island to the east you just explored. The next island over to the west, a vertically oriented island, houses a Guarded Treasure event. Disembark near the center of the island and make your way to shore, where you'll have to fight about a half dozen Drowned Dead (level eighteen). Once they're dead make your way south to find some ruins. climb a ledge to find the remains of a grisly shrine near which is a chest.
Further west are several more islands, some of which are worth exploring. There are two circular islands immediately to the west, the southern of which has Endrega Drones (level twenty) infesting its western coast, while some Bandits (levels thirteen, fourteen and sixteen) dwell on the south-eastern part of the island. On the other hand, on the north-eastern part of the northern island you can find a chest, while on the south-western shore are a variety of parcels to loot. If you dive off the coast near these parcels and activate your Witcher Senses you should be able to spot a chest on the sea floor. Hmm... tough call, a bunch of unguarded treasure, or monsters and no treasure?
The next island to the west - the last and western-most island in this island chain - actually consists of three islands which are connected by shallow, fordable water. On the largest of these islands (the one with three peninsulas sticking out of its northern edge) you'll find several Harpies (level nineteen). On the northern beach along this island, between the eastern and central peninsula, you'll find numerous crates worth looting, while on a grassy hill west of the island's eastern beach you'll find another chest. Next cross some shallow water to the western-most island, which is liberally littered with parcels as well as a chest on the beach and a chest on a grassy promontory.
Smugglers Cache (Level 16)
Board your boat again and sail to the north-west to find the last two map markers that make up this exploration. The first (southern-most) of these two events is just north-west of a tiny, sliver of an island south-west of a larger island. Anyways, go to the first event - a Smugglers Cache - and kill some Ekhidna (level sixteen) then loot the floating chests.
Sail north and slightly east to find a Hidden Treasure event off the western coast of the larger island to the north. Dive off your ship near a beached ship and swim to the northern side of said ship. When you reach the shore, find a ledge you can scale north of the ship, head south-east a bit, then turn south-west to find a gap in the ship's railing, the deck along which has that convenient white paint that screams "you can climb this ledge!". Helpful, that. Get a running start and jump onto the ship, or... well, you could just jump from the beach onto the ship. Still, who doesn't like a good running jump? Head over to the cabin and blast the door with Aard, then loot a corpse and a variety of containers inside the cabin to complete this event.
You're now done with this bit of exploration, only one more to go; it's time to explore from the waters off of south-eastern Ard Skellig to the island of Faroe.