If mermaids did exist, wouldn't they be the ugliest human like beings on earth? Think about it, if a human lived underwater, gravity won't take as much effect on them therefore they will become bloated, overweight and with sagging features? And of course they can't just limit what they eat cause they need to eat a lot to create enough fat(better yet, blubber) to keep them warm in freezing cold ocean waters and to be able to have enough strength to swim.