i just saw ur post on the discord and yes that is the same glitch, pretty sure the person that posted it on reddit did it before me

so i came across the Bleeding Mark settlement which is flooded prior to completing the Deluge side quest, and noticed that there is a campfire marked on the map thats supposed to be in that settlement. however, that campfire isnt actually there when the settlement is still flooded. i wondered what would happen if you were to fast travel to that campfire, so i tried. this spawned me under the ground, in a pool of water that extends out under the settlement and around it. i think the cause might be something to do with weird collision with the wood beams that extend onto and through the ground. there is a specific wooden "spike" that touches the ground quite close to where you spawn when fast traveling there. i havent found a way to get back in bounds, and havent really done extensive testing other than figuring out how far out the water extends horizontally (quite far, almost all the way to scalding spear). i have no idea if this could be useful at all, especially since it seems very difficult to get up through the surface, and there isnt a whole lot going on under the map in the area you can access besides the outside of some underwater sub-areas, big rocks and a massive void (obviously) underneath the water.

just thought i'd share my findings.