How many times have gamers gone on adventures where they are either on a ship that gets sunk and they get rescued by mermaids or mermen, or they are on a ship and get attacked by some Kuo-Toa or Sahuagin or even drown zombies of previous sailors?

Well, all of these creatures (except for maybe the zombies) have to have some sort of culture under the sea or ocean. So how about a challenge where we map out an underwater kingdom or city?