In a medium-size saucepan, bring the water to a boil and take off heat. Stir in the Jell-O until dissolved. Place over low heat, add the marshmallow and stir until marshmallows are melted in. Remove from heat. Pour into a two-quart bowl. Mix in drained crushed pineapple, cottage cheese, and salad dressing. Place salad in refrigerator to cool and firm.