In the scene where Ariel gets her voice back and Ursula bursts out of her dress and grabs Ariel when she drags her over the side Ariel is still wearing that blue dress but in the next scene when Triton confronts Ursula Ariel has lost the dress. Also notice that If she was wearing a dress like the blue one then in the era that we are lead to believe the story is set in (Wooden ships, Washer women, Horses and Carts) then wouldn't Ariel be wearing a corset? There is no sign of this underwater and she is wearing her shells again.