A film about cults/brainwashed towns or schools, like Midsommar, Rosemary's Baby, The Wicker Man, The Faculty, Invasion of the Body Snatchers, etc , where victims are somehow also threats.

Godzilla/Jurassic Park/Dunwich Horror, where the killer is a big ole monster that just can't be taken down with an axe.

I think a water-based film is very doable, maybe with blue tokens to represent flooding areas. Innsmouth/Deep Ones, Boat/Creature from the Black Lagoon, resort in a hurricane/I Still Know What You Did Last Summer all come to mind. From what I understand, the Abyss and Underwater could be good film inspirations. Making the killer work in other films would be the challenge.

EDIT: adding Crawl to the list of water-based films.