Bring the water and salt to a boil in a large pot. Add the prawns to the water and return to a boil; cook at a boil for 5 minutes. Remove the prawns with a strainer and set aside.
Cook the beans, eggplant, and bok choy in the water until slightly tender, about 3 minutes. Drain and reserve the liquid. Set the vegetables aside.
Heat the olive oil in a large skillet over medium heat; cook and stir the onion and garlic in the hot oil until fragrant, about 5 minutes. Sprinkle the achiote powder over the mixture; stir until you produce an even orange-red color. Add the peanut butter and continue stirring until the peanut butter has melted evenly into the mixture. Stir the reserved water into the mixture and bring to a boil; cook at a boil for 3 minutes before stirring in the prawns and vegetables. Continue boiling together 2 minutes more before serving.