So imagine a world set in 1939-1946 (around the ww2 era) But the mythical creatures at sea, such as mermaids, krakens, the island-sized turtle and the like are very much thriving and present in that world, and sometimes work actively against humans (EG. Mermaids still trying to get ships to wreck and krakens attacking shipping convoys for food...)

I still don't know the premise or the plot but I'd like your opinions on it... This also gives me an idea for things to build once the plot solidifies.