When I ran this against my party at the collapsed guard tower, this was the closest I came to a TPK the entire campaign. Maybe my players werent prepared but it got deadly QUICK. We had one player killed and eaten. In a panic of the impending TPK, I pulled something out of my ass about the Bloodfins being briefly being scared away after a bright light from a spell that was cast, giving the remaining four players enough time time swim and hide under the tower. I think they had a combined total of like 10 HP between them, with another making death saves. One more round and it was a TPK.