During my Sweden trip last year I have made pics from the Halmstad harbor, estuary of Nissan River. I could imagine that this structures have been breakwater lights and now are not active?
Postion of DSCO1808 :56.661011, 12.854232 . See the old bicycle!
I can't imagine something else than old breakwater lights. But seems you're discoverer, i don't see them in any list.
Maybe this is the other old and inactive breakwater light!?
I would assume so