OK, I think I missed an area. I went through a full playthrough and never saw the Lurkers or Seastrider Kai--I'm told that they are in a "cave" and "underground", but the closest thing I can find is the Underground River area under the dungeon at the Glistening Streams, and that seems empty.
(Also, is there a way to find a key for the door at the back of the fisherman's shack that sometimes appears in the Glistening Streams?)