Amaterasu cannot use them until she tosses a Mermaid Coin into them. Her first Mermaid coin is obtained in Sasa Sanctuary from Mr. Bamboo after she battles Gi. After this, Mermaid Coins are available for sale from merchants or can be found in treasure chests. However, after Amaterasu obtains Fountain, she can teleport between Mermaid Springs without using a Mermaid Coin.