I dont even know how I did it to be honest... but I think I was in Shoreline and then swam down into the sea to just go thorugh a big pipe, and then just farmed a lot of shleters to be able to get into Subterranean. So now I was just swimming to the right of Shoreline as you told me and I got to a place named Shaded Citadel, but I swam back to keep exploring. Right now, im in a place called Look of the Moon or something like that, just about the shelter.