Walkthrough for the Treasure Hunt Hidden Treasure: Applied Escapology in The Witcher 3 Blood and Wine.
Due west of the Beauclair Port signpost you’ll find a pair of islands, on the northern of which you’ll find Beauclair Prison (the signpost for which is on the southern of these two islands). On the western end of the northern island that houses the prison you’ll find a tree, under which you’ll find some remains which you can loot for an Escape Artist’s Journal. Pick it up and read it to start this Treasure Hunt.
Return to the water and swim (swim, not sail) north to the quest marker for “Applied Escapology”), along the way plundering a chest and a sack near some ruins. Dive at the quest marker to find a tunnel you can swim into (Killer Whale might be useful here) which leads to a subterranean section of the prison. There are plenty of skeletons to loot, and a few containers as well, but keep your Witcher Senses active and look for the red-glowing skeleton, which possesses a Masterly Ornamented Key - the device you’ll need to score even more treasure shortly. Most of all, don’t be lured by the apparent sunlight to the northeast, you can’t surface for air there. Dive as many times as you need to in order to obtain whatever loot you wish, but when you at least have the key, note that the quest marker has changed, directing you back south - just south of the search area for the quest Turn and Face the Strange, in fact.
Get back in your ship and sail south to the new quest marker and prepare yourself for another dive. The first thing you’ll need to do when you’re in the water is dispatch some more Drowners (level forty), which pollute the water here. Once they’re no longer a threat, dive under the water and swim towards the “Entrance” marker near the southern shore of the lake. Pass through the elegant elven entrance and shortly beyond you’ll find a hole in the ground you can swim deeper into. Do so and swim south to find a cavern full of treasure. More importantly, there’s a pocket of air above this chamber you can surface at for air, so plundering all the loot here shouldn’t be nearly as risky as getting the key way.
Loot the container that glows red under your Witcher Senses (which requires the “Masterly Ornamented Key”) to complete the quest Applied Escapology. There’s lots of other trinkets to pick up, too, including chests, urns, and piles of Crowns that are yours for the taking. Well worth a bit of a detour.