Clean octopus by removing the eyeballs and beak. Massage the skin with salt for 20 minutes. In a large pot, bring water to boil and add sake, vinegar and soy sauce. Bring to boil and turn down the heat to low. Gently place the cleaned octopus into the pot by holding its head and slowly lowering it into the liquid, legs first. Place a lid one size smaller than the pot on top of the octopus. Put a weight on the lid to stabilize. Cook for 20 minutes and then flip the octopus. During the cooking process, skim off the froth that rises to the top of the pot. Cook for another 20 minutes and remove from heat. Cut legs into ¾ inch slices. Place seven pieces on a plate with a pinch of sea salt on the side.