I've been trying to remember the title and author of this one. It would have either been on LiveJournal or Ao3.

It is probably wincest, but I am not completely sure of that. The boys are staying at a place by the sea, I believe, for a case. The "monster" turns out to be a human male who disguises himself as a creature and intentionally scares a young girl badly enough that she falls into the sea. Sam dives in to save her and he ends up smashed into some rocks by the waves and damamging (permanently?) an appendage (I think it was his hand/arm). He has to cling to the rocks and be pulled out.

Any help would be appreciated.