Ok, this is what I think when Regina told Rumble that she had a mermaid problem that is because when Regina tried to kill Snow again Snow must have been in the boat or by water and Regina tried to drown Snow. Ariel comes to Snow’s rescue and saves Snow.
Or since Regina is Ursula, Ursula tried to drown Snow in her boat and the Mermaid (Ariel) saves Snow from Ursula 🙂
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